Seven Ukrainian soldiers were killed and seven wounded when they were ambushed by pro-Russian separatists near the eastern Ukrainian town of Kramatorsk, the Ukrainian defence ministry said.
It was the biggest single loss of life by the Ukrainian army since soldiers were sent into the mainly Russian-speaking east of the country to break up armed separatist groups who have seized control of towns and public buildings to push their demands for autonomy.
The ministry, in a statement published on its website, said an armoured column came under fire as it approached a bridge near a village 12 miles from Kramatorsk, one of several hot spots in the region.
About 30 rebels, who had taken cover among bushes along a river, attacked with grenade-launchers and automatic weapons, immediately killing two soldiers and wounding three others.
“In all, as a result of the prolonged fighting, six members of the armed forces were killed. Eight soldiers were wounded, one of them seriously,” the statement said.
The state security service said later that the seriously wounded soldier had died while being transported to hospital.
Before the Kramatorsk incident, the Ukrainian Defence Minister Mikhail Koval said a total of nine servicemen had been killed so far in the army’s “anti-terrorist” operation, which has been directed mainly against rebels in the towns of Slovyansk and Mariupol. The dead included five pilots who died when their helicopters were shot down.
In pictures: The Ukraine crisis
In pictures: The Ukraine crisis
1/39 Ukraine crisis
Ukrainian soldiers from the 'Azov' battalion guard their base in Mariupol, Donetsk
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Children were transferred from orphanages in Donetsk and Makeyevka to escape the fighting between the Ukrainian army and pro-Russian separatists
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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
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A view inside the Youth Culture Centre destroyed by pro-Russian separatists in Donetsk
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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
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A boy ascends the stairs of a bomb shelter after the shelling in the Petrovskiy district in Donetsk
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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
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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
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Medical workers carry a wounded pro-Russian militant on a stretcher after armed clashes occured between pro-Russian gunmen and Ukrainian troops in Slavyansk
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A member of a newly-formed pro-Russian armed group called the Russian Orthodox Army mans a barricade near Donetsk airport
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Black smoke rises from a shot down Ukrainian Army helicopter outside Slovyansk
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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
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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
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A bloodstained icon of Jesus is seen among blood soaked shattered glass atop a wrecked truck near the Donetsk airport
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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
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A woman reacts after seeing the lifeless body of a man killed by shrapnel following a shelling from Ukrainian government forces in Slovyansk
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Pro-Russian militants take position on the roof of the international airport of the eastern Ukrainian city of Donetsk
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A woman embraces a pro-Russian separatist from the "East" battalion during a rally in the eastern city of Donetsk
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A Ukrainian helicopter Mi-24 gunship fires its cannons against rebels at the main terminal building of Donetsk international airport
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A pro-Russian gunman changes his position near the airport, outside Donetsk
21/39 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
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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!"
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An elderly woman leans against the chest of a pro-Russian gunman in Lenin square in Donetsk
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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
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Ukrainian troops stand guard at a checkpoint on the road near the eastern city of Izum, Donetsk
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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
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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
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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
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Members of a local election commission sort ballots as they start counting votes of referendum on the status of Luhansk region in Luhansk
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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
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Members of a local election commission empty ballot boxes as they start counting votes after a referendum, at a polling station in Lugansk
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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
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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
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Voters visit a polling station to take part in the referendum on the status of Donetsk
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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
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A statue of Lenin is placed in front of a pro-Russian barricade on the outskirts of the eastern Ukrainian town of Slavyansk
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A feamle veteran (C) sings during 'Victory Day' celebrations in Donetsk
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People stand near the burning Mariupol police station
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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
Meanwhile Russia responded to Western sanctions against its activities in Ukraine by saying it would refuse to co-operate with the US in the International Space Station.
The Deputy Prime Minister Dmitry Rogozin said Moscow would reject a US request to prolong the orbiting station’s use beyond 2020, and bar Washington from using Russian-made rocket engines to launch military satellites.
Moscow took the action, which also included suspending operation of GPS satellite navigation system sites on its territory from June, in response to US plans to deny export licences for hi-tech items that could help the Russian military.
“We are very concerned about continuing to develop hi-tech projects with such an unreliable partner as the United States, which politicises everything,” Mr Rogozin told a news conference.
Washington wants to keep the $100bn, 15-nation space station project in use until at least 2024, four years beyond the previous target.
While six years away, the plan to part ways on a project which was supposed to end the “space race” underlines how relations between the former Cold War rivals have deteriorated since Russia annexed Ukraine in March.
Since the end of the US Space Shuttle project, Russian Soyuz spacecraft have been the only way astronauts can get to the space station, whose crews include both Americans and Russians.
Mr Rogozin said Moscow was planning “strategic changes” in its space industry after 2020 and aims to use money and “intellectual resources” that now go to the space station for a “project with more prospects”. He suggested Russia could use the station without the United States, saying: “The Russian segment can exist independently from the American one. The US one cannot.”
The US space agency Nasa is working with companies to develop space taxis with the goal of restoring US transport to the station by 2017, but the US currently pays Russia more than $60m per person to fly its astronauts up.
In addition to the hi-tech sector sanctions, the US and the EU have imposed visa bans and assets freezes on officials and politicians and targeted companies with links to President Vladimir Putin.