A communal bus drives past burnt cars near a checkpoint that was the scene of an overnight gunfight near the city of Slaviansk April 20, 2014. Gunfire erupted near the makeshift checkpoint manned by pro-Russian separatists near Slaviansk in eastern Ukraine early on Sunday, killing at least two people.

Slavyansk - Pro-Kremlin rebels in east Ukraine appealed Sunday for Russian “peacekeepers” to sweep in after a deadly gunfight killed at least two of their militants, shattering an Easter truce and sparking “outrage” in Moscow.

But the Western-backed authorities in Kiev claimed the violence was a set-up by Russia to create a pretext for it to send troops in.

The attack, near the flashpoint town of Slavyansk, undermined an accord worked out in Geneva between Russia, Ukraine and Western powers on Thursday that demanded “illegal armed groups” surrender their weapons and cease occupations of public buildings.

The deal was aimed at easing what has become the worst crisis between Washington and Moscow since the end of the Cold War, but now appears to have stalled.

Russia has tens of thousands of troops massed on Ukraine's border in what NATO says is a state of readiness to invade, while the United States, according to The Washington Post, is preparing to send ground troops to neighbouring Poland.

Sunday's gun battle occurred in the early hours in a village 18 kilometres (11 miles) west of Slavyansk.

Vladimir, a masked 20-year-old pro-Russian rebel claimed to be at the scene of the shootout, told AFP: “Four cars pulled up to our roadblock around 1:00am (00:00 SA time Saturday). We wanted to conduct a check, and then they opened fire on us with automatic weapons.”

He said three of the militants were killed.

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 leader of the separatist rebels in Slavyansk, Vyacheslav Ponomaryov, said he believed two of the attackers were also killed.

He declared a midnight-to-6:00am (23:00 SA time to 05:00 SA time) curfew in Slavyansk - and appealed for Russian President Vladimir Putin to send in Russian troops as “peacekeepers to defend the population against the fascists”.

Putin has said he “very much hopes” he will not have to send his forces into Ukraine, but he insists he has a “right” to do so.

On Sunday, Russia's foreign ministry declared its “outrage” at the deadly attack, blaming the deaths of “innocent civilians” on ultra-nationalists who were at the vanguard of the street protests that forced the February ouster of Ukraine's pro-Kremlin president Viktor Yanukovych.

The ministry said locals had found the attackers' cars containing weapons, satellite maps and business cards belonging to the ultra-nationalist group Right Sector.

It demanded Kiev abide by the Geneva accord.

But a Right Sector spokesman told AFP that Russia's claims were “lies” and “propaganda” designed to portray the east as ungovernable for Kiev.

Ukraine's government, confirming three people were killed, described the latest violence as a “cynical provocation” by Russian-armed separatists.

Interior Minister Arsen Avakov said he was heading to the east to inspect troops in the region.

Western-backed authorities in Kiev had announced they were suspending military operations to oust the rebels over Easter, which ends Monday, but the gunfight ended days of relative calm.

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.

The stalled implementation of the Geneva agreement threatened to deepen the crisis.

With pro-Kremlin rebels refusing to comply with its terms, Washington has been ratcheting up 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 progress was made on the ground.

He also is preparing to send ground troops to Ukraine's neighbour Poland, to expand NATO's presence in eastern Europe, according to a report in The Washington Post newspaper.

Amid of the Cold War-style tensions, the Organisation for Security and Cooperation in Europe (OSCE), which is 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 “there won't be many more chances for a peaceful solution.”

The sudden spike in tensions put paid to attempts by some ordinary Ukrainians to embrace Easter as a time of peace across their country.

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 overnight when the Orthodox leaders in Kiev and Moscow traded barbs.

Kiev's Patriarch Filaret thundered to the faithful that Russia was an “enemy” whose “attack” on Ukraine was doomed to failure because it was evil and contrary to God's will.

In Moscow, 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 Sunday, Ukrainian Prime Minister Arseniy Yatsenyuk lashed out at Putin for having a “dream to restore the Soviet Union”.

Washington has warned Moscow that Ukraine is in a “pivotal period” and that progress was needed on the Geneva accord “within days”.

US Vice President Joe Biden is scheduled to make a visit to Kiev on Tuesday. - Sapa-AFP