Ukraine and Russia have traded allegations following a deadly clash in eastern Ukraine.
The Ukrainian Interior Ministry said at least three people were killed and three others injured in the shooting incident at a makeshift checkpoint outside Slovyansk early on April 20.
Russian state-run media reported that five people had died, including three pro-Russia activists.
The identity of the attackers was not immediately clear.
Russia's Foreign Ministry expressed "outrage" over the incident, saying it proved that Ukrainian authorities do not wish to disarm "nationalists and extremists."
Pro-Russian authorities in the region presented documents and other materials that they alleged showed that the Ukrainian far-right nationalist group Right Sector was responsible.
Right Sector denied any involvement and instead accused Russian special services.
The Ukrainian Security Service said no Ukrainian organization was involved. It denounced the incident as a "provocation" staged by outsiders.
Local pro-Russia separatist leader Vyacheslav Ponomarev, the self-declared mayor of Slovyansk, has announced a curfew and appealed to Russian President Vladimir Putin to send Russian peacekeepers to the region.
Including Slovyansk, pro-Russia separatists are reported to remain in control of public facilities in around 10 areas of mainly Russian-speaking eastern Ukraine.
Monitors from the Organization for Security and Cooperation In Europe (OSCE) were expected in eastern Ukraine on April 20 to oversee the implementation of a new international plan aimed at disbanding illegal armed groups and ending occupations of public buildings.
The deal was signed in Geneva on April 17 by the European Union, Russia, Ukraine, and the United States.
The progress of the initiative was not immediately clear. The Ukrainian government wants to see pro-Russia groups broken up, while pro-Russia activists say Ukrainian protester groups also need to halt their occupations of public property.
The latest developments came ahead of the expected visit to Kyiv on April 22 of U.S. Vice President Joe Biden, who is due to hold talks with Ukrainian leaders.
The United States has been a strong backer of Ukraine’s new authorities. Washington has warned it could impose more sanctions against Russia unless Moscow takes action to de-escalate the situation in Ukraine.
In an interview with the U.S. television network NBC, which was broadcast on April 20, Ukrainian Prime Minister Arseniy Yatsenyuk renewed accusations that Russia is undermining global stability by interfering in Ukraine.
Yatesenyuk accused Putin of trying to restore the Soviet Union by moving into former Soviet territories.
This was rejected as “"false" by the Russian ambassador to the United States, Sergei Kislyak.
In his Easter sermon at the Vatican, Roman Catholic Pope Francis called for an end to violence in Ukraine.
The pope asked God to "enlighten and inspire the initiatives that promote peace in Ukraine."
The pontiff’s message came after the Ukrainian and Russian Orthodox Christian leaders used their Easter remarks to exchange accusations
over the crisis.
With reporting by Reuters, AP, and ITAR-TASS