Pro-Russian separatists controlling a city in eastern Ukraine have called on Moscow to deploy peacekeepers to the area.
It follows a gun battle at a makeshift checkpoint in which at least three people died.
Russia's foreign ministry expressed outrage at the fatal shooting incident which it blamed on Ukrainian nationalists.
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 Slaviansk.
Ukraine officials said three died in a clash between what they described as "two groups of citizens".
The attack will test an already fragile international accord that is supposed to defuse Ukraine's armed stand-off.
Hours earlier, Ukraine's Western-backed government in Kiev had declared a truce to coincide with the Easter religious holiday.
It was hoped the truce would give 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.
It risks escalating this week with a US 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.
Separatist militiamen near the eastern Ukrainian city of Slaviansk told Reuters that a convoy of four vehicles had approached their checkpoint at around 11pm (Irish time) and opened fire.
"We had three dead, four wounded," one of the separatist fighters, called Vladimir, told Reuters at the checkpoint, where there were two burned-out jeeps.
He said the separatists returned fire and killed two of the attackers, who he said were members of Right Sector, a group loosely aligned with the government in Kiev.
Slaviansk has been controlled by separatists since last weekend.
A Reuters cameraman at the scene said he saw the bodies of two local fighters.
One of the dead was dressed in camouflage fatigues, the other in civilian clothes.
The deaths were the first in armed clashes in eastern Ukraine since the Geneva accord was signed on Thursday.
A senior mediator with the Organisation for Security and Cooperation in Europe (OSCE) set off for eastern Ukraine today to try to persuade pro-Russian groups to lay down their arms, in line with the Geneva accord.
But the attack near Slaviansk is likely to harden their position that they will not lay down their arms because they believe they are under threat of violent attack from forces loyal to Kiev.
The United States government has said that it will impose further sanctions on Moscow if, by early this week, there is no measurable sign that the pro-Russian separatists are complying with the accord.