US Vice President Joe Biden will begin a two-day visit to Ukraine on Monday, hours after a fragile Easter truce was shattered and pro-Kremlin rebels in the country's east appealed for help from Russian "peacekeepers".

Biden's arrival in Kiev comes after a deadly gunfight killed at least two pro-Russian militants, an incident which sparked "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 in troops.

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 Russia and the West since 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.

Vladimir, a masked 20-year-old pro-Russian rebel, said Sunday's shootout erupted when four cars pulled up to a roadblock manned by the separatists in the early hours.

"We wanted to conduct a check, and then they opened fire on us with automatic weapons," Vladimir said.

Three of the separatists were killed, he added.

An AFP photographer saw two bodies laid out in a truck near the scene.

The identity of the assailants, who escaped before the pro-Russian rebels could bring in reinforcements, was not known.

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

He declared a midnight-to-6:00 am curfew in Slavyansk, and appealed for Russian President Vladimir Putin to send in troops as "peacekeepers to defend the population against the fascists" -- the separatists' label for Ukraine's new government and its supporters.

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

Moscow blames ultra-nationalists

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 overthrow of Ukraine's pro-Kremlin president Viktor Yanukovych in February.

The ministry said locals had found the attackers' cars containing weapons, satellite maps and business cards belonging to the ultra-nationalist group Pravy Sektor ("Right Sector"). It demanded the Ukrainian government 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 by 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 on Sunday to inspect troops in the region, said investigations into the shootout were ongoing.

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 on 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 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

Pope Francis pleaded for peace in Ukraine in his Easter Sunday 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 Christian 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 against 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 on Sunday, Ukrainian Prime Minister Arseniy Yatsenyuk lashed out at Putin for having a "dream to restore the Soviet Union".

"The world has a reason to be concerned about Putin's intention because what (the) Russia Federation did, they undermined the global stability," Yatsenyuk told NBC's "Meet the Press".

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