Ukraine is edging towards "the point of no return", a senior UN official says, amid rising tensions between security forces and pro-Russia separatists.
UN Assistant Secretary General for Human Rights Ivan Simonovic told the BBC that the crisis had worrying echoes of the 1990s war in his native Croatia.
Reports from eastern Ukraine say clashes between government forces and separatist militants have continued.
The separatists have not taken part in EU-brokered talks to defuse the crisis.
Overnight, attacks in the east wounded four Ukrainian servicemen, the ministry of defence in Kiev was quoted as saying by Ukrainian media.
Separatist militants are said to have attacked a military camp near the city of Izyum, in Kharkiv region, and four checkpoints near Sloviansk in Donetsk region.
On Saturday, the separatists appointed a prime minister for what they call the People's Republic of Donetsk.
The man, Alexandr Borodai, said the self-proclaimed entity would apply to join Russia.
The separatists have taken control of government buildings across cities in south-eastern and southern Ukraine.
Violence between the two sides has left dozens of people dead in recent weeks.
A new Ukrainian president is due to be elected on 25 May.
Mr Simonovic told the BBC: "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 countless incidents of abduction, torture and murder in south and eastern Ukraine.
Although abuses have been committed on both sides, Mr Simonovic said, the majority were by the separatists.
He added that he hoped the planned presidential election could take place, but that it would be "extremely difficult".
"I firmly believe that there is a window of opportunity that should be used," he said, but added that it was closing.
The separatists have held referendums in the regions of Donetsk and Luhansk, and claimed to have won overwhelming popular backing. Both the Ukrainian government in Kiev and Western countries regard the votes as illegitimate.
The BBC's Mark Lowen in Donetsk says both sides in the conflict are digging in.
The Ukrainian government refuses to talk to armed separatists and the self-proclaimed authorities say they will not hold a dialogue until Kiev ends its "occupation" of the east.
The revolt in the east gained momentum after Russia annexed Ukraine's mainly ethnic Russian region of Crimea in March.
Moscow acted after the overthrow of Ukraine's elected pro-Russian President Viktor Yanukovych during unrest in the capital Kiev in February.
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