Russia says it is "outraged" by a gun battle in restive eastern Ukraine that saw three pro-Russian militants killed, shattering the Easter truce.
A deadly gunfight in restive east Ukraine has shattered a fragile Easter truce, with Russia declaring it was "outraged" at the return to violence in the crisis-hit former Soviet republic.
Three pro-Russian militants and one attacker were killed early on Sunday in a firefight at a roadblock close to the separatist-held town of Slavyansk, a local pro-Kremlin rebel leader, Vyatcheslav Ponomarev, said.
Vladimir, a masked 20-year-old pro-Russian rebel who was at the barricade, told AFP: "Four cars pulled up to our roadblock around 1am (0800 AEST). We wanted to conduct a check, and then they opened fire on us with automatic weapons."
He said there were about 20 attackers, and confirmed the three rebel deaths, but was not sure of casualties on the other side.
An AFP photographer saw the bodies of two militants laid out in a truck near the scene.
The identity of the assailants, who escaped before militant reinforcements arrived, was not known.
The Ukrainian interior ministry confirmed there was an "armed clash", but gave a toll of one dead and three injured. It said police were investigating.
The gunfight broke days of relative calm.
Western-backed authorities in Kiev had announced they were suspending military operations to oust the rebels over Easter, which ends on Monday. The last deadly clash was last Thursday, when three pro-Russian militants were killed by Ukrainian soldiers when they tried to attack a military base in the southeast port city of Mariupol.
Russia's foreign ministry said Moscow was "outraged at this provocation by the fighters" and urged Kiev to abide by an accord signed in Geneva on Thursday by Ukraine, Russia, the United States and the European Union.
Moscow blamed Sunday's deaths of those it called "innocent civilians" on the Right Sector, an extreme-right group that was at the vanguard of protests that ousted Ukraine's pro-Kremlin president Viktor Yanukovych in February.
But Right Sector spokesman Artyom Skoropadsky in Kiev dismissed the charge as "a clear provocation by the Kremlin".
The deputy head of Ukraine's National Security and Defence Council, Viktoria Syumar, said Moscow was orchestrating the violence.
The stalled implementation of the Geneva agreement threatened to deepen the worst East-West crisis since the Cold War.
With pro-Kremlin rebels refusing to comply, Washington has been increasing pressure on Moscow, which it sees as pulling the strings in the Ukrainian insurgency.
US President Barack Obama has threatened to impose more sanctions on Moscow if no de-escalation occurs, and is preparing to send ground troops to Ukraine's neighbour Poland, a report in The Washington Post says.
Russia has tens of thousands of troops massed on Ukraine's eastern border in what NATO fears is a state of readiness to invade.
The Organisation for Security and Co-operation in Europe (OSCE), tasked with monitoring the Geneva accord, said it was sending a high-ranking team to east Ukraine on Sunday.
German Foreign Minister Frank-Walter Steinmeier urged all parties to abide by the Geneva agreement, warning in an interview with newspaper Bild am Sonntag that "there won't be many more chances for a peaceful solution".
Pope Francis also pleaded for peace in his Sunday Easter prayer. "We ask you to enlighten and inspire the initiatives that promote peace in Ukraine," he prayed.
But efforts to that end were undermined when the Orthodox religious leaders in Kiev and Moscow traded barbs overnight.
Kiev's Patriarch Filaret thundered that Russia was an "enemy" whose "attack" on Ukraine was doomed to failure because it was evil and contrary to God's will.
The patriarch of the Russian Church, Kirill, led a prayer for Ukraine in which he called on God to put "an end to the designs of those who want to destroy Holy Russia". Kirill said Ukraine was "spiritually and historically" at one with Russia, even if politically separate, and he prayed it would soon have leaders who were "legitimately elected".
Russia refuses to recognise the authority of Kiev's pro-Western government.
In comments to be broadcast on US television on Sunday, Ukrainian Prime Minister Arseniy Yatsenyuk lashed out at Russian President Vladimir Putin for having a "dream to restore the Soviet Union".
Putin has blown hot and cold on the crisis, from threatening, saying he "hoped" he would not have to invade Ukraine, to conciliatory, saying "no obstacle" existed to better relations with the West.
US Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov that "full and immediate compliance" was needed within "the next few days".
US Vice President Joe Biden is scheduled to visit Kiev on Tuesday.
Meanwhile, a poll conducted by Kiev's Institute for International Sociology suggested just over half of the Ukrainians in the eastern Donetsk region were against coming under Russian rule again, while over a quarter were in favour.