Russia’s foreign ministry has expressed outrage at a fatal shooting incident in eastern Ukraine which it blamed on Ukrainian nationalists, the BBC reports.
Russian state media reported that five people were killed in a gun attack on a checkpoint manned by pro-Russian activists near the town of Sloviansk.
Ukrainian officials said one died in a clash between “two groups of citizens”.
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 Organization 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 said in a statement on its Facebook page that it had nothing to do with the events in Sloviansk, which bore the hallmarks of a provocation by Russia’s Federal Security Service.
Ukraine’s interior ministry said that details were still being established, but that one person was killed and three others were taken to hospital with gunshot wounds.
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.
But 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.
Meanwhile in an interview to be aired in full later on Sunday on NBC’s Meet the Press, Ukraine’s interim Prime Minister Arseniy Yatsenyuk accuses Russian President Vladimir Putin of trying to “restore the Soviet Union”.
If Mr Putin succeeded, Mr Yatsenyuk says, it would be “the biggest disaster of this century”.
Ukraine has been in crisis since President Viktor Yanukovych was toppled in February.
Russia then annexed the Crimean peninsula – part of Ukraine but with a Russian-speaking majority – in a move that provoked international outrage.
The annexation followed a referendum in Crimea that backed a move to join the Russian Federation.
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 decentralization 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.