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Another team of international observers has been captured in eastern Ukraine as fighting between government troops and rebels continues.
Five members of the Organisation for Security and Co-operation in Europe (OSCE) have been unaccounted for since 7pm on Thursday.
In a statement, the OSCE said the four international monitors and a Ukrainian translator were travelling in two vehicles in Severodonetsk, north of Luhansk, when they were stopped by "armed men".
It was not immediately clear who detained them or why but pro-Russian rebels have claimed responsibility for kidnapping another team of OSCE observers on Friday .
An insurgent leader in the Donetsk region told journalists that members of the first group were in their custody on Thursday.
The rebels said they would “deal with this and then release them,” but did not estimate how long that would take.
OSCE observers have increasingly become a target for rebels as clashes with the Ukrainian army continue and 11 more were imprisoned at an occupied television studio in Donetsk for seven hours on Wednesday.
Show all 66 1 /66In pictures: The Ukraine crisis In pictures: The Ukraine crisis Ukraine crisis Ukrainian soldiers from the 'Azov' battalion guard their base in Mariupol, Donetsk
EPA
In pictures: The Ukraine crisis Ukraine crisis Children were transferred from orphanages in Donetsk and Makeyevka to escape the fighting between the Ukrainian army and pro-Russian separatists
Reuters
In pictures: The Ukraine crisis Ukraine crisis Ukrainian servicemen captured by pro-Russian separatists sit on the ground as they are assigned to clean a street in Snizhne in the Donetsk region
Reuters
In pictures: The Ukraine crisis Ukraine crisis A view inside the Youth Culture Centre destroyed by pro-Russian separatists in Donetsk
Getty
In pictures: The Ukraine crisis Ukraine crisis A woman holds a portrait of her dead son as she speaks during a rally in front of Ukrainian President Petro Poroshenko's office in Kiev
AFP
In pictures: The Ukraine crisis Ukraine crisis A boy ascends the stairs of a bomb shelter after the shelling in the Petrovskiy district in Donetsk
AP
In pictures: The Ukraine crisis Ukraine crisis A medic looks at thirty coffins prepared for the funerals of pro-Russian rebels killed during heavy fighting at Donetsk airport, outside a Donetsk morgue
In pictures: The Ukraine crisis Ukraine crisis Relatives mourn near the coffin of Mark Zverev, a taxi driver shot dead during clashes at the Donetsk airport between Ukrainian troops and the pro-Russian rebels, during his funeral in the village of Grabari on the outskirts of Donetsk
In pictures: The Ukraine crisis Ukraine crisis Medical workers carry a wounded pro-Russian militant on a stretcher after armed clashes occured between pro-Russian gunmen and Ukrainian troops in Slavyansk
In pictures: The Ukraine crisis Ukraine crisis A member of a newly-formed pro-Russian armed group called the Russian Orthodox Army mans a barricade near Donetsk airport
In pictures: The Ukraine crisis Ukraine crisis Black smoke rises from a shot down Ukrainian Army helicopter outside Slovyansk
AP
In pictures: The Ukraine crisis Ukraine crisis A woman is overcome with emotion as she looks at blood stains and damage around a wrecked truck of supporters of the self-proclaimed 'Donetsk People's Republic' on road leading to the Donetsk International Airport
In pictures: The Ukraine crisis Ukraine crisis A local man looks at damage near a wrecked truck of supporters of the self-proclaimed 'Donetsk People's Republic' on road leading to the Donetsk International Airport
In pictures: The Ukraine crisis Ukraine crisis A bloodstained icon of Jesus is seen among blood soaked shattered glass atop a wrecked truck near the Donetsk airport
In pictures: The Ukraine crisis Ukraine crisis A family member attends a funeral for Olga Prokhorenko (60) who was killed by shrapnel after Ukrainian government forces shelled their location, during the funeral in Slovyansk
In pictures: The Ukraine crisis Ukraine crisis A woman reacts after seeing the lifeless body of a man killed by shrapnel following a shelling from Ukrainian government forces in Slovyansk
In pictures: The Ukraine crisis Ukraine crisis Pro-Russian militants take position on the roof of the international airport of the eastern Ukrainian city of Donetsk
In pictures: The Ukraine crisis Ukraine crisis A woman embraces a pro-Russian separatist from the "East" battalion during a rally in the eastern city of Donetsk
In pictures: The Ukraine crisis Ukraine crisis A Ukrainian helicopter Mi-24 gunship fires its cannons against rebels at the main terminal building of Donetsk international airport
In pictures: The Ukraine crisis Ukraine crisis A pro-Russian gunman changes his position near the airport, outside Donetsk
In pictures: The Ukraine crisis Ukraine crisis A pro-Russian gunman aims his weapon near the airport, outside Donetsk. Ukraine's military launched air strikes against separatists who had taken over the airport in the eastern capital of Donetsk in what appeared to be the most visible operation of the Ukrainian troops since they started a crackdown on insurgents
In pictures: The Ukraine crisis Ukraine crisis Pro-Russians ride on a truck in Donetsk. A convoy of an armored personnel carrier and seven trucks carrying several hundred heavily armed men drove through central Donetsk and gunmen got out of the trucks, stood to attention and gave shots in the air in jubilation as a crowd of several thousand supporters cheered them and chanted: "Heroes!"
In pictures: The Ukraine crisis Ukraine crisis An elderly woman leans against the chest of a pro-Russian gunman in Lenin square in Donetsk
In pictures: The Ukraine crisis Ukraine crisis Pro-Russian militants guard a psychiatric hospital after shell explosions during the fighting between pro-Russian militants and the Ukrainian army, in Semyonovka village, outside Slavyansk
In pictures: The Ukraine crisis Ukraine crisis Ukrainian troops stand guard at a checkpoint on the road near the eastern city of Izum, Donetsk
In pictures: The Ukraine crisis Ukraine crisis Members of the Democratic Alliance party take part in a performance in front of the French embassy in Kiev. Protesters set up a pool with the blood of animals and models of warships in a protest against French plans for the sale of two Mistral helicopter carriers to the Russian Navy
In pictures: The Ukraine crisis Ukraine crisis A man carries a portrait of pro-Russian activist Vadim Hudich, who was killed in a shooting incident at the head of a funeral procession in the eastern Ukranian town of Krasnoarmeisk
In pictures: The Ukraine crisis Ukraine crisis A cyclist rides near of pro-Russian barricade in Slaviansk
In pictures: The Ukraine crisis Ukraine crisis Ukrainian soldiers stand guard at a checkpoint not far from Slaviansk
In pictures: The Ukraine crisis Ukraine crisis People celebrate as they gather for the announcement of the referendum results on the status of the Republic of Lugansk, at the square before the monument of Shevchenko in Lugansk
In pictures: The Ukraine crisis Ukraine crisis Pro-Russian armed men stand guard as people hold a rally to mark and celebrate the announcement of the results of the referendum on the status of Luhansk
In pictures: The Ukraine crisis Ukraine crisis A Pro-Russian armed militant stands guard as a man casts his vote during a referendum organized by the so-called Donetsk People's Republic members, near of their checkpoint in Slavyansk
In pictures: The Ukraine crisis Ukraine crisis A woman is led aside as her partner is detained and taken away by Ukrainian security forces for being aggressive at an army checkpoint during a referendum on self-rule in the southeastern port city of Mariupol
In pictures: The Ukraine crisis Ukraine crisis A voter is seen inside a voting booth at a polling station during the referendum on the status of Donetsk region in the eastern Ukrainian city of Donetsk
In pictures: The Ukraine crisis Ukraine crisis Members of a local election commission sort ballots as they start counting votes of referendum on the status of Luhansk region in Luhansk
In pictures: The Ukraine crisis Ukraine crisis Members of a local election commission empty a ballot box as they start counting votes of the referendum on the status of Donetsk region in Donetsk
In pictures: The Ukraine crisis Ukraine crisis Members of a local election commission empty ballot boxes as they start counting votes after a referendum, at a polling station in Lugansk
In pictures: The Ukraine crisis Ukraine crisis Local residents watch as others give first aid to a man who was shot in the leg by Ukranian militia in the village of Krasnoarmisk. Eyewitness said that Ukranian militia tried to stop the referendum voters briefly taking the City Hall of Krasnoarmisk, where unarmed pro-Russian supporters were gathering. Reportedly the Ukranian militia came out out the building and started shooting at the people, killing at least one man and leaving at least two others injured
In pictures: The Ukraine crisis Ukraine crisis Local people take part in a referendum organized by the so-called Donetsk Republic members, at a polling station in Mariupol
In pictures: The Ukraine crisis Ukraine crisis Residents of eastern Ukraine began voting on an independence referendum that was organized by pro-Russian separatists and rejected by the government in Kiev. Russian-speakers and supporters of Moscow have been rallying in the region since March, when a referendum on independence led to Russia's annexation of Crimea
In pictures: The Ukraine crisis Ukraine crisis Members of a local election commission count votes after a referendum organized by the so-called Donetsk People's Republic members at a polling station in Donetsk
In pictures: The Ukraine crisis Ukraine crisis A nurse casts her vote into the mobile ballot box carried by a referendum helper while elder patients look on in a hospital corridor, during the referendum organized by the so-called Donetsk Republic members in Mariupol
In pictures: The Ukraine crisis Ukraine crisis An armed pro-Russian activists stands guard near a ballot box during a referendum organized by the so-called Donetsk People's Republic members, at a polling station in Donetsk
In pictures: The Ukraine crisis Ukraine crisis A Ukrainian serviceman speaks on a phone at a checkpoint outside the southeastern port city of Mariupol
In pictures: The Ukraine crisis Ukraine crisis A man ticks his ballot inside a polling booth at a polling station during a referendum in the eastern Ukrainian city of Slavyansk
In pictures: The Ukraine crisis Ukraine crisis Voters visit a polling station to take part in the referendum on the status of Donetsk
In pictures: The Ukraine crisis Ukraine crisis An Armoured Personnel Carrier (APC) with a Russian flag drives through the center of Slaviansk during the day of referendum organized by the so-called Donetsk People's Republic members in Slavyansk
In pictures: The Ukraine crisis Ukraine crisis A statue of Lenin is placed in front of a pro-Russian barricade on the outskirts of the eastern Ukrainian town of Slavyansk
In pictures: The Ukraine crisis Ukraine crisis A feamle veteran (C) sings during 'Victory Day' celebrations in Donetsk
In pictures: The Ukraine crisis Ukraine crisis Fire fighters tackle the fire in the Mariupol police station in the Mariupol police station
In pictures: The Ukraine crisis Ukraine crisis People stand near the burning Mariupol police station
In pictures: The Ukraine crisis Ukraine crisis Russians and Ukrainians hold Ukrainian and Russian national flags during a protest outside the European Union office in Athens
In pictures: The Ukraine crisis Ukraine crisis Multi-medalled local World War II veteran Alexander Ionov stands alongside a T-34 tank, which rolled up in front of the Ukraine Security Service barricades in Lugansk
In pictures: The Ukraine crisis Ukraine crisis Pro-Russia separatists carry the coffin of one of their comrades during the funeral of three combatants and a civilian who were killed in clashes between Ukrainian forces and pro-Russia separatists at the town of Slaviansk
In pictures: The Ukraine crisis Ukraine crisis Pro-Russian activists hold a Russian tricolor flag as they gather outside the Regional Interior Ministry building in Lugansk
In pictures: The Ukraine crisis Ukraine crisis An armed pro-Russian activist in casual street wear instead of military camouflage stands on the steps of the Regional Interior Ministry building in Lugansk
In pictures: The Ukraine crisis Ukraine crisis The self-styled mayor of Luhansk region Valery Bolotov (C) holds an ignition key for a "Tigr" in Luhansk. The Tigr, a military light armoured vehicle, was donated by the Liberal Democratic Party of Russia to pro-russian activists
In pictures: The Ukraine crisis Ukraine crisis Pro-Russian activists gather on the steps of the Regional Interior Ministry building in Lugansk. Pro-Russian forces in Ukraine have been pressing for the referendum in the provinces of Lugansk and Donetsk, arguing that they want to be independent from Ukraine and merged with a greater Russia
In pictures: The Ukraine crisis Ukraine crisis A man (C) who appears to be a member of the media as he wears a press badge stands between pro-Russian activists and speaks to a crown outside the Regional Interior Ministry building in Lugansk
In pictures: The Ukraine crisis Ukraine crisis Ukrainian soldiers stand guard beside an armoured personnel carrier at a checkpoint in Mariupol. Ukrainian forces seized the rebel-held city hall in the eastern port city of Mariupol, driving out pro-Russian activists, then withdrew, making no attempt to hold onto the building
In pictures: The Ukraine crisis Ukraine crisis Armed pro-Russian protesters take up positions by the railway lines, near Slaviansk
In pictures: The Ukraine crisis Ukraine crisis Four pro-Russian militiants lay on the ground with hands tied after being arrested on suspicion of the implementation of MANPADS (Man-portable air-defense systems) shots to a helicopter of the Ukrainian forces at a checkpoint near Slavyansk. Ukraine's defence ministry raised the toll during an assault on the rebel-held town of Slavyansk to two of its helicopters shot down and two servicemen killed. Authorities say the pro-Russian insurgents used shoulder-launched rockets to bring down the two Mi-24 helicopters. Earlier, the interior minister said one of those killed was a helicopter pilot
In pictures: The Ukraine crisis Ukraine crisis A woman kisses the shroud during a funeral of a man, who was killed during clashes between Ukrainian and pro-Russian forces in Slaviansk
In pictures: The Ukraine crisis Ukraine crisis Relatives react during a funeral of a man, who was killed during clashes between Ukrainian and pro-Russian forces in Slaviansk
In pictures: The Ukraine crisis Ukraine crisis A referendum on independence for eastern Ukrainian independence needs to be delayed and peace restored in the region, said Russian President Vladimir Putin even as government and separatist forces clashed anew
In pictures: The Ukraine crisis Ukraine crisis Local people and relatives weep during a funeral, in Slaviansk, Ukraine, 07 May 2014, of a man, who was killed during clashes between Ukrainian and pro-Russian forces in Slaviansk
In April, pro-Russian separatists captured seven military observers from the organisation near Slovyansk, accusing them of being “Nato spies”.
Four Germans, a Dane, a Pole and a Czech were detained in the town of Kramatorsk before being released at the start of May.
The OSCE’s special monitoring mission to Ukraine started earlier this year at the request of the government and with the agreement of all 57 member states.
It aims to reduce tension and foster peace and security following Russia's annexation of Crimea and an insurgency in southern and eastern parts of Ukraine.
At least 14 soldiers died on Thursday when rebels shot down a helicopter carrying Ukrainian troops near Slovyansk.
The Government in Kiev had vowed to crack down on militias who have seized official buildings in several cities.
President Petro Poroshenko, who will be sworn in to office next week, previously said the campaign against rebels, who he compared to Somali pirates, would last “hours”.
Additional reporting by AP
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