Ukraine's police and intelligence service accused Russia of staging a fatal shooting incident on Sunday in which pro-Moscow separatists were killed in the east of the country.
"Armed lawbreakers and saboteurs who are terrorising the local population around Slaviansk ... have turned to cynical provocation,'' the SBU security service said in a statement, describing the incident as a "staged attack."
No group was present "other than the saboteurs and crime figures, supported and armed by officers of Russia's GRU" military intelligence, the SBU added. The Ukrainian nationalist group Right Sector, blamed by separatists for the shooting, denied involvement and also accused Russian special forces.
Russia has accused Ukraine of failing to protect civilians in the Russian-speaking east and has threatened to intervene on their behalf - as it did before annexing Crimea last month.
The Ukrainian Interior Ministry noted that its police were unable to operate in Slaviansk since pro-Moscow gunmen took over the town earlier this month. But in a statement it said it had established that at least three men were killed in what it suspected might have been an incident set up by Russian agents.
Separatists manning a checkpoint on a road into Slaviansk near the town of Bylbasovka returned fire, it said, after gunmen in four cars drove up and shot at them at 2:20 a.m. (2320 GMT).
Three separatists were killed, two locals and a third as yet unidentified. Three were wounded. About a dozen attackers took off in two cars, carrying an unknown number of dead and wounded.
The Interior Ministry said none of its forces had mounted an operation overnight around Slaviansk, which it described as "the most dangerous place in Ukraine, in view of the presence in the town of foreign saboteurs and illegal armed groups".
"At the same time," it added, "One cannot but suspect the speed with which camera crews from Russian TV stations appeared at the scene of the shooting, and the obviously staged subject matter of news reports in the Russian media."
It noted that the news was broken by media run by Dmitry Kiselyov, a journalist who is close to Russian President Vladimir Putin and who, the ministry noted, is the subject of visa bans imposed by the United States and EU as part of sanctions aimed at punishing Russia for annexing Crimea.
Hours before the shootings, Ukraine's Western-backed government in Kiev had declared a truce to coincide with the Easter religious holiday, giving international mediators an opportunity to try to persuade armed pro-Russian groups to disarm.
The crisis over Ukraine has dragged relations between Moscow and the West to their lowest ebb since the Cold War, and risks escalating this week with a U.S. threat of further sanctions against Moscow if the pro-Russian separatists do not pull back.
A deal signed in Geneva last week by the European Union, Russia, Ukraine and the United States agreed that illegal armed groups would go home, but so far they have shown no signs of budging, leaving the accord looking threadbare.
The crisis in Ukraine began late last year when President Viktor Yanukovich turned his back on closer ties with Europe, prompting protests in the capital.
They led to him fleeing Kiev, and a pro-Western interim administration taking over.
Soon after, Moscow used its military to back separatists in Ukraine's Crimea Peninsula, before Russian President Vladimir Putin signed a document annexing Crimea. The United States and European Union responded by slapping sanctions on Russian officials.
The crisis has now shifted to the east of Ukraine, where armed groups of pro-Russian separatists have seized public buildings, saying they reject Kiev's rule. Adding to the tension, Russia has shifted extra troops to the border with Ukraine.
Russia argues that it acted only to protect ethnic Russians in Ukraine from an administration in Kiev which they say was installed in an illegal coup d'etat, which they allege has links to the far-right and discriminates against Russian speakers.
After a meeting on Saturday in the Ukrainian capital with diplomats from the four parties to the Geneva accord, Swiss envoy Christian Schoenenberger, whose country is chair of the OSCE, said its monitors had spoken to several activists occupying public buildings.
"For the time being the political will is not there to move out," he said.
"That's the task of the monitors, to create this political will, inform the people, so eventually they will understand that the best option for them is to move out," he told reporters.
The Ukrainian foreign ministry had promised that, as a gesture of goodwill for the Easter holiday, it would suspend the active phase of an operation it had launched to re-assert its authority in the east of the country.
In another sign of reconciliation, the Interior Ministry issued an Easter message which asked members of the ministry's disbanded Berkut unit to help defend Ukraine's unity.
In the days when Yanukovich was clinging to power in Kiev, Berkut members are alleged to have been responsible for shooting dead dozens of protesters.
Their unit was disbanded, and some of them have joined the pro-Russian separatists.
However, many people in eastern Ukraine, which has a large Russian-speaking population, feel deep suspicion of the new authorities in Kiev.
The separatists say they will not leave the buildings they have occupied until the pro-Kiev protest groups occupying Independence Square - scene of the months-long protests against Yanukovich, also go home.
They want guarantees that they will be given a large degree of autonomy from Kiev and that protections of their rights will be enshrined in a new constitution.
At Easter church services in Kiev and in Moscow, senior clergymen issued sharply contrasting appeals for peace.
"In these Easter days our prayers to God are for the people of Ukraine, for a reconciliation of enmity, for an end of violence, for people's love for each other, so that they should not be divided," Patriarch Krill, head of the Russian Orthodox Church, said in a recorded video message.
In his Easter message, Patriarch Filaret, head of Ukraine's Orthodox Church, condemned what he described as Russia's aggression against his country.
"God cannot be on the side of evil, so the enemy of the Ukrainian people is condemned to defeat," he said. "Lord, help us resurrect Ukraine."