Pro-Kremlin rebels in east Ukraine appealed yesterday for Russian "peacekeepers" after a deadly gunfight killed at least two of their militants, shattering an Easter truce and sparking "outrage" in Moscow.
The Western-backed authorities in Kiev claimed the violence was a set-up by Russia to create a pretext for it to send its 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 agreement 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.
Watch: Pope makes Easter plea for Ukraine peace after gun battle
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 US, according to The Washington Post, is preparing to send ground troops to neighbouring Poland.
Yesterday's gun battle occurred in the early hours in a village 18 kilometres west of Slavyansk.
Vladimir, a masked 20-year-old pro-Russia rebel who claimed to be at the scene of the shootout, said: "Four cars pulled up to our roadblock around 1am. We wanted to conduct a check, and then they opened fire on us with automatic weapons."
He said three of the militants were killed.
The identities of the assailants, who escaped before militant reinforcements arrived, were 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-6am 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.
Russia's foreign ministry declared its "outrage" at the deadly attack, blaming the deaths of "innocent civilians" on ultranationalists who were at the vanguard of the street protests that forced the ousting 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 ultranationalist group Right Sector. It demanded Kiev abide by the Geneva accord.
A Right Sector spokesman said 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. The 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 today. The gunfight ended days of relative calm.
The stalled implementation of the Geneva agreement threatened to deepen the crisis.
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.
Amid the tensions, the Organisation for Security and Co-operation in Europe, tasked with monitoring the Geneva accord, said it was sending a high-ranking team to east Ukraine yesterday. Germany's foreign minister, Frank-Walter Steinmeier, urged all parties to abide by the Geneva agreement, warning in a newspaper interview: "There won't be many more chances for a peaceful solution."