* At least three dead after clash at separatist checkpoint
* Ukraine nationalists deny role, blame Russian special forces
* Separatist mayor calls for Russia to send peacekeepers
* Also calls curfew in town (Adds mutual accusations, colour)
By Aleksandar Vasovic
SLAVIANSK, Ukraine, April 20 (Reuters) - Pro-Russian authorities in eastern Ukraine presented identity documents, maps and a business card to support their view that Ukrainian far-right nationalists had carried out a deadly attack early on Sunday.
The nationalists and authorities in Kiev said it was a staged display to conceal the hand of Russian secret services in the gunfight, in which at least three people were killed.
The truth, as in so much of the standoff over Ukraine between Moscow and Kiev, was hard to establish.
Reuters journalists saw two dead bodies - one in combat fatigues and one that of a known local man in civilian clothes - lying in a truck some hours after the reported incident. It was not clear whether they were attackers, defenders or bystanders.
There were also two burned out jeeps at the makeshift checkpoint near Bylbasovka, some 18 km (11 miles) outside the town.
The local pro-separatist mayor of the nearby town of Slaviansk announced a curfew and urged Moscow to send in peacekeepers, complicating efforts to defuse the standoff between Ukraine's pro-Western leaders and Russia.
Pro-Russian separatists said attackers had driven up to the barricaded checkpoint in the early hours and begun shooting.
They said three of their number had been killed and in a subsequent exchange of fire two of the attackers were killed. They said the attackers were from the nationalist Right Sector.
It was the first time members of Right Sector, which was at the forefront of pro-Western demonstrations in Kiev, have been accused of involvement in any deadly violence in the east, where Russia has repeatedly said it fears civil war.
A spokesman for the group denied any role.
The exchange of charge and counter-charge between separate armed groups, Moscow and Kiev underscored the deep gulf of suspicion and mistrust even as the international community moved to give teeth to a four-way agreement in Geneva last week to de-escalate tension in eastern Ukraine.
Armed separatists are in control of Slaviansk, one of 10 or so points in the Russian-speaking east of Ukraine where localised rebellions have broken out against Kiev's pro-Western leaders following the overthrow of Kremlin-backed president Viktor Yanukovich two months ago.
A personal appeal by Vyacheslav Ponomaryov, the self-appointed pro-separatist mayor of Slaviansk, to Russia's president Vladimir Putin to consider sending in peacekeeping forces was certain to raise alarm in Kiev.
"Vladimir Vladimirovich, fascists and imperialists are using all means to try to defeat our small provincial city in the Donetsk Region", he said in remarks directed to Putin and reported by Russian online news channel LifeNews.
Ponomaryov, who was installed by separatists when they took over the town on April 12, announced a curfew for the town to run between the midnight and 6 a.m.
MOSCOW, KIEV BLAME-GAME
Russia's foreign ministry issued a statement backing the separatists' version of events and blaming armed members of Right Sector, an ultra-nationalist group which played a key role in the overthrow of Moscow-backed president Viktor Yanukovich in February.
It was a source of surprise, the ministry said, that the incident had happened after Russia, the European Union, the United States and Ukraine had signed the April 17 accord in Geneva calling on people to desist from using violence or intimidation.
In Kiev, Right Sector denied any involvement and blamed Russia's special forces, whom the Kiev leadership suspects of being behind separatist rebellion in the east, of organising the attack.
Ukraine's state security agency (SBU) also denounced what it called a "cynical" provocation by criminals and agents "who are supported and armed by Russian military intelligence".
And Kiev's interior ministry caustically commented on the speed "with which camera crews from Russian TV stations appeared at the scene of the shooting ..."
The incident occurred as monitors from the OSCE human rights organisation, in a follow-up move to the Geneva agreement, moved into eastern Ukraine to attempt to persuade separatists to put down their arms and come out of state buildings they have occupied in the region.
Klaus Zillikens, a German diplomat and head of the OSCE mission Donetsk, emphasised the difficulty of establishing the truth in a region where rumour spreads quickly and where the big Russian-speaking population is influenced by news reports from across the border.
He told Russian radio station Ekho Moskvy that his experience in the region was that reports of casualties were often exaggerated or hard to pin down. He gave the example of a Ukrainian government statement last week that its forces had killed three militants. "We could only confirm one," he said.
A separatist fighter, who gave his name as Vladimir, said Right Sector members drove up to the checkpoint outside Slaviansk at around 2 a.m. "One of our men approached them. They shot him in the head and he fell immediately. There was some sniper fire. We had three dead and four wounded."
He said the separatists brought in reinforcements and in subsequent shooting two of the attackers were killed.
Local authorities later showed personal identity cards of people alleged to belong to Right Sector, a night-sight for a rifle, some maps and the business card of Dmytro Yarosh, the group's leader.
A Right Sector spokesman in Kiev poured scorn on the display. "They say that visiting cards of Dmytro Yarosh were found. It's surprising that Right Sector members carry around his visiting cards and they don't get destroyed when the car burns up," Artem Skoropadsky told Reuters.
One of the two bodies at the scene was that of a man in combat fatigues with the black and orange St George's ribbon, a Russian insignia for military valour. A second man, whom local people said was well known in the area, wore civilian clothes and had gunshot wounds in the face.
Local people laid flowers and touched the bodies in a gesture of farewell.
"This is going on for too long, and I want to see it over. Kiev should stop sending its mercenaries to attack peaceful people. We are people of Donetsk and we should be left alone," said Marina, in her 50s.
Alexei, a 73-year-old pensioner, said local people were frightened. "I do not want to know who was shooting at who, I just want this to end so we can continue to live like normal people," he said. "I do not care if this is Ukraine or Russia, I just want armed people out of our town." (Additional reporting by Dmitry Madorsky and Gleb Garanich in Slaviansk, and Natalia Zinets in Kiev; Writing by Richard Balmforth; editing by Philippa Fletcher)