An agreement to avert wider conflict in Ukraine was faltering yesterday, 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, where he is expected to announce a package of technical assistance. The visit is likely to be more important as a symbol of support than for any specific promises Biden makes in public.

“He will call for urgent implementation of the agreement reached in Geneva last week while also making clear ... that there will be mounting costs for Russia if they choose a destabilising rather than constructive course in the days ahead,” a senior administration official told reporters.

Russia, Ukraine, the European Union and the United States signed off on the agreement in Geneva on Thursday designed to lower tension in the worst confrontation between Russia and the West since the Cold War. The agreement calls for occupied buildings to be vacated under the auspices of envoys from the Organisation for Security and Cooperation in Europe.

We did not negotiate, we talked

But no sooner had the accord been signed than both sides accused the other of breaking it, while the pro-Moscow rebels disavowed the pledge to withdraw from occupied buildings.

An OSCE mediator, Mark Etherington, held his first meeting with the leader of separatists in Slaviansk, a town which rebels have turned into a heavily-fortified redoubt. Etherington said he had asked the pro-Russian self-proclaimed “people’s mayor” of the town, Vyacheslav Ponomaryov, whether he would comply with the Geneva agreement, but gave no hint about the response.

Ponomaryov later told a news conference: “We did not negotiate, we talked. We told them our position, what happened here, and they told us about their plans.”

Etherington said he had also asked about people being held in Slaviansk, including the woman who was serving as mayor until the uprising. Her fate has not been made clear.

In other signs the Geneva accord was far from being implemented, activists in Slaviansk brought up trucks laden with sand and were filling sandbags to reinforce their barricades. In nearby Kramatorsk, local media showed masked gunmen taking over the office of the SBU security service in the town, and leading away a civilian identified as the local police chief.

Separatists told Reuters they would not disarm until Right Sector, a Ukrainian nationalist group based in Western Ukraine, did so 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 member of a separatist militia. “We need dialogue. This is not dialogue. It is monologue.”

Russia says Right Sector members have threatened Russian speakers. Kiev and Western countries say the threat is largely invented by Russian state-run media to justify Moscow’s intervention and cause alarm in Russian speaking areas.

Moscow blames Right Sector for a shooting on Easter Sunday morning, when at least three people were killed at a checkpoint manned by armed separatists. Right Sector denies involvement, while Kiev said Russia provoked the violence.

US Secretary of State John Kerry urged his Russian counterpart Sergei Lavrov yesterday to help implement the Geneva deal, including by “publicly calling on separatists to vacate illegal buildings and checkpoints”, spokeswoman Jen Psaki said.

In its account of their telephone conversation, the Russian Foreign Ministry said Lavrov had called on Kerry to “influence Kiev, not let hotheads there provoke a bloody conflict” and to encourage it “to fulfil its obligations unflaggingly”. One European diplomat said the deal was a way for Russian President Vladimir Putin to buy time and undermine momentum towards tougher sanctions.

The Slaviansk separatists released around a dozen Ukrainian soldiers in blue uniforms on Monday, without making clear the circumstances under which they had been held. Gordik said armoured vehicles that were surrendered by a column of Ukrainian paratroops last week would stay in the town.

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