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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 Organization for Security and Cooperation in Europe.
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 on Monday 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: "Talks and compromises are just part of his tactics," said the diplomat. "He wants to have Ukraine."
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.
Putin announced last month that Moscow has the right to intervene in its neighbours to protect Russian speakers. He then annexed Ukraine's Crimea peninsula.
Moscow has since massed tens of thousands of troops on the Ukrainian border, and Kiev and its Western allies say Russian agents are directing the uprising in the east, including the "green men" - heavily armed, masked gunmen in unmarked uniforms.
In his latest move, likely to be seen by the West as a further threat to the post-Cold War order, Putin signed a law on Monday making it easier for Russian speakers across the former Soviet Union to obtain Russian citizenship.
Eastern Ukraine is largely Russian-speaking and many residents are suspicious of the pro-European government that took power in Kiev in February when Moscow-backed President Viktor Yanukovich fled the country after mass protests.
Separatists have declared an independent "People's Republic of Donetsk" in the east's biggest province and have named themselves to official posts in towns and cities, setting up checkpoints and flying Russian flags over government buildings.
There is also some support for Ukrainian unity in the region, but pro-Kiev activists have had a lower profile since the separatists took up arms.
One activist who helped organise a unity rally in Rubizhne, a town in the eastern Luhansk region, told Ukraine's Channel 5 television that separatists attacked it, forcing the rally to disperse. Local police said a policeman was hurt when unidentified people tried to disrupt the rally.
In the regional capital Luhansk, Interfax-Ukraine news agency said a meeting of about 3,000 people in the local SBU headquarters had elected a "people's governor" and voted to hold a two-stage referendum next month on union with Russia.
Ukraine announced an operation to retake rebel-held territory earlier this month, but that modest effort largely collapsed in disarray.
Kiev has declared an "Easter truce", though it is far from clear it could muster any real force if it tried. The army is ill-equipped, untested and untrained for domestic operations, while the government in Kiev doubts the loyalty of the police.
The United States and EU have imposed visa bans and asset freezes on some Russians over the annexation of Crimea, measures explicitly designed not to have wider economic impact and which have been mocked as pointless by Moscow.
Washington and Brussels both say they are working on tougher measures they will impose unless Russia's allies in eastern Ukraine back down, although building a consensus is tricky in Europe where many countries rely on Russian energy exports.
The OSCE, a European security body that includes both NATO members and Russia, has so far deployed around 100 monitors and mediators in Ukraine and expects their number to rise.
An OSCE spokesman said the mediators were visiting separatist-occupied buildings with copies of last week's Geneva accord to explain it to the people inside.
"It's a mixed experience dealing with checkpoints and so forth and there is a varying reaction to teams. There is a hardened attitude in Donetsk or Slaviansk but some other areas are more accommodating," spokesman Michael Bociurkiw said. "When teams go to smaller centres people are more willing to talk." (Additional reporting by Natalia Zinets, Alastair Macdonald and Jeff Mason in Kiev, Dmitry Madorsky in Slaviansk, Alissa de Carbonnel in Donetsk, Doina Chiacu in Washington, Steve Gutterman and Vladimir Soldatkin in Moscow; writing by Peter Graff and Philippa Fletcher; editing by David Stamp)