Ukraine is on the brink of chaos, a senior UN official has warned.
UN Assistant Secretary General for Human Rights Ivan Simonovic told the BBC that Ukraine is approaching “the point of no return”, as there continues to be no resolution to the crisis.
Mr Simonovic said that tensions between government forces and pro-separatists had worrying echoes of the 1990s war in Croatia, where he was born.
Over the weekend the Russian foreign ministry issued a statement condemning what it described as a sharp escalation of violence in eastern Ukraine, where pro-Russian militants have declared “people's republics” in Donetsk and Luhansk, following an unofficial referendum in favour of independence.
The separatists have refused to participate in EU-brokered talks being held in Kharkiv in a bid to resolve the crisis, saying they are willing to discuss only the withdrawal of Ukrainian troops and the recognition of the independence of the regions.
On Saturday, a man named Alexander Borodai was appointed as prime minister for the so-called People’s Republic of Donetsk, the BBC reported.
In pictures: The Ukraine crisis
In pictures: The Ukraine crisis
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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
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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
Mr Simonovic told the broadcaster: “What I'm really afraid is that country is approaching to a point of no return if there is no adequate and urgent action taken.”
The UN says it has documented innumerable episodes of abduction, torture and murder in the south and east of the country.
And Mr Simonovic said that although these incidents were committed by both sides in the conflict, the majority were by the separatists,
An election for the new Ukrainian president is scheduled for 25 May, following the ousting of Russia-backed President Viktor Yanukovych in February, after months of protests in Kiev.
Mr Simonovic added that although he hoped the election would go ahead it would be “extremely difficult”.
“I firmly believe that there is a window of opportunity that should be used,” he said.
On Saturday lawmakers and officials from eastern Ukraine poured criticism on the fledging central government, accusing it of ignoring legitimate grievances of the regions which have been overrun by separatists.
But acting Prime Minister Arseniy Yatsenyuk called on the eastern leaders to resist the armed men and support the government's efforts to devolve powers to the regions.
No representatives of the separatists from Donetsk and Luhansk were present at the talks.
Oleksandr Bandurka, a Communist party lawmaker and police general from central Ukraine, said that these negotiations make no sense because “we're not talking to those who oppose us. We cannot ignore them”.
Ukraine's first president, Leonid Kravchuk, who is chairing the talks, angrily reacted that “no one in the world talks to killers and terrorists. Putin doesn't talk to terrorists”.
Russia has pushed for the federalization of Ukraine, since that would allow Moscow to retain influence over areas in Ukraine dominated by Russian-speakers. Many in eastern Ukraine and in the capital favour closer ties to Europe and fear being pulled back into Moscow's orbit.
Volodymyr Groisman, acting prime minister in charge of the reforms to decentralize the government, countered the claims of some eastern lawmakers that only federalization will bring peace to Ukraine.
“You were saying the unitary system of government is no longer effective?” Groisman said.
“An inefficient government and dirty politics — this is what led to the fact that so many people in our country are poor.”
Attempting to end the talks on a conciliatory note, Yatsenyuk quoted Ukrainian poet Taras Shevchenko and told the leaders from eastern Ukraine: “We are ready to embrace you and hope that you are too.”
US State Department spokesperson Jen Psaki rejected the “illegal actions” of pro-Russian militants. But she said the US supports the efforts by “elected and legitimate representatives” meeting in Kharkiv “to discuss constitutional and nonviolent approaches to resolving their differences”.
“Any decisions made about Ukraine must be taken by those with lawful authority, representing the citizens of Ukraine as a whole, and not under threat of foreign military intervention,” she said.
The next round of talks is expected to be held in the central city of Cherkasy on Wednesday.
Additional reporting by Associated Press