An agreement reached last week to avert wider conflict in Ukraine was faltering, with pro-Moscow separatist gunmen showing no sign of surrendering government buildings they have seized.

US and European officials say they will hold Moscow responsible and impose new economic sanctions if the separatists do not clear out of government buildings they have occupied across swathes of eastern Ukraine over the past two weeks.

US Vice President Joe Biden arrived in Kiev yesterday, where he is expected to announce a package of technical assistance. But the visit is likely to be more a symbol of support than for any specific pledges.

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Russia, Ukraine, the European Union and the US signed off on an agreement in Geneva on Thursday designed to lower tension in the worst confrontation between Russia and the West since the Cold War.

It calls for occupied buildings to be vacated under the auspices of envoys from the Organization for Security and Cooperation in Europe.

But no sooner had the accord been signed than both sides accused the other of breaking it.

An OSCE mediator held his first meeting with the leader of separatists in Slaviansk.

Russian separatists said they would not disarm until Right Sector, a Ukrainian nationalist group, disarms first. "Who should surrender weapons first? Let us see Right Sector disarm first, let them make the first step and we will follow," said Yevgeny Gordik, a separatist militia.

Moscow blames Right Sector for a deadly shooting on Easter Sunday morning, when at least three people were killed at a checkpoint. Right Sector denies involvement.