Russia has expressed outrage at a fatal shooting in eastern Ukraine which it blamed on Ukrainian nationalists.
At least three people were reported killed in a gun attack on a checkpoint manned by pro-Russian activists near the town of Sloviansk.
A Ukrainian official said it may have been a shootout between criminals. Nationalists have denied involvement.
The incident comes as pro-Russian groups continue to occupy government buildings defying a deal to leave.
Ertogrul Apakan, who heads the special mission of the Organisation for Co-operation and Security in Europe in Kiev, said his deputy would be in Donetsk to try to get them to comply with an agreement reached on Thursday to ease the crisis.
Russia, Ukraine, the EU and US agreed during talks in Geneva on Thursday that illegal military groups in Ukraine must be dissolved, and that those occupying government premises must be disarmed and leave.
But the separatists' spokesman in the city of Donetsk said that the Kiev government was "illegal", and vowed they would not go until it stepped down.
TV pictures showed what was described as the aftermath of an attack on a pro-Russian checkpoint at about 01:00 local time (22:00 on Saturday GMT), including the body of a man under a cover.
The BBC is unable to verify the footage. However, a Reuters journalist at the scene reported seeing two bodies in a truck.
Daylight Reuters TV footage of the scene shows several burnt-out vehicles.
The Russian foreign ministry said the Ukrainian far-right group Right Sector was behind the attack. A business card with the name of its leader Dmytro Yarosh appeared in the unverified Russian TV pictures.
"Russia is indignant about this provocation by gunmen, which testifies to the lack of will on the part of the Kiev authorities to rein in and disarm nationalists and extremists," it said in a statement.
Right Sector spokesman Artyom Skoropadsky told the BBC the group had nothing to do with the shooting.
"Right sector was not there, and whatever happened there was an obvious provocation from the Russian secret services," he said.
Viktoriya Siumar, deputy head of Ukraine's National Security Council, also speaking to the BBC, said the shooting was being investigated, but said there were indications that it was "an argument between local criminal groups".
"The level of criminality in eastern Ukraine increased substantially recently," she added.
This is the first fatal incident in the region since Thursday's agreement, prompting Sloviansk rebel leader and self-proclaimed mayor Vyacheslav Ponomarev to call on Russian President Vladimir Putin to send peacekeepers. He also asked for food and weapons.
Mr Ponomarev added that a "people's army of Donbass" was being set up. Donbass (Don river basin) is the industrial area of eastern Ukraine made up of Donetsk and Luhansk regions.
The BBC's James Reynolds in Donetsk says that the Geneva deal is already in trouble and events in Sloviansk will do little to change that.
'Threat to the globe'
Meanwhile in an interview for NBC's Meet the Press aired on Sunday, Ukraine's interim Prime Minister Arseniy Yatsenyuk accused Russian President Vladimir Putin of trying to "restore the Soviet Union".
"It's crystal clear that for today, Russia is the threat to the globe, and the threat to the European Union, and a real threat to Ukraine," Mr Yatsenyuk said.
Ukraine has been in crisis since President Viktor Yanukovych was toppled in February.
Russia then annexed Crimea following a regional referendum that approved joining the Russian federation. The annexation provoked international outrage.
Pro-Russian activists then occupied buildings in several eastern Ukrainian cities, many calling on Moscow to support them.
Ukraine has said that operations against the pro-Russian militants have been suspended over Easter.
Ukraine's interim authorities have appealed for national unity and promised to meet some of the demands of pro-Russian protesters.
These include the decentralisation of power and guarantees for the status of the Russian language.
But the US has warned the next few days will be pivotal and has threatened more sanctions against Russia if it fails to abide by the agreement.
US Vice-President Joe Biden is set to visit Kiev on Tuesday.
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