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 under which "illegal armed groups" were to surrender their weapons.

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

Russia has an estimated 40,000 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 a village 18 kilometres (11 miles) west of Slavyansk.

Vladimir, a masked 20-year-old pro-Russian rebel who said he was at the scene of the shootout, told AFP: "Four cars pulled up to our roadblock around 1:00am (2200 GMT 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-6:00 am (2100 GMT to 0300 GMT) curfew in Slavyansk -- and appealed for Russian President Vladimir Putin to send in Russian troops as "peacekeepers to defend the population against the fascists".

Later, he said: "If you can't send peacekeeping forces, send us weapons."

- Moscow blames ultra-nationalist group -

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

On Sunday, Russia's foreign ministry declared its "outrage" at the deadly attack.

It blamed the deaths of the "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 that 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, who travelled to the east Sunday to inspect troops in the region, said investigations were ongoing into the shootout.

- Geneva accord stalled -

The gunfight ended days of relative calm underpinned by a promise by the Western-backed authorities in Kiev to suspend military operations to oust the rebels over Easter.

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.

But with the pro-Kremlin rebels refusing to comply with the Geneva accord, 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 is made on the ground.

A Kremlin spokesman shrugged off as "absurd" claims that Washington could sanction Putin directly, after an article in Britain's The Times newspaper cited anonymous sources saying the United States could target Swiss bank accounts belonging to the leader that allegedly hold some $40 billion (29 billion euros).

Russia's ambassador to the United States, Sergei Kislyak, told Fox News that sanctions represented a return to the "Cold War mentality" but said Moscow could "withstand pressures".

- Prayers for peace -

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 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 calling on God to put "an end to the designs of those who want to destroy Holy Russia" and pleading for Ukraine to soon have "legitimately elected" leaders.

In comments 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.