John Kerry, the US secretary of state, threatened Russia with further sanctions unless the country stopped backing separatists in eastern Ukraine as he acknowledged that the release of seven international observers taken hostage a week ago was a step forward.
Mr Kerry told Sergei Lavrov, Russia's foreign minister, that it was critical for Russia "to withdraw support from separatists and to assist in removing people from Ukrainian government buildings", according to the AP news agency.
The telephone conversation came as a mob of several hundred people led by masked men stormed state security headquarters in Donetsk in eastern Ukraine.
The Russian foreign ministry said Mr Lavrov had told his counterpart that the US should use its influence to urge Ukraine's government to immediately halt a military offensive in southeast Ukraine that had entered a second day on Saturday, Reuters reported.
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In Donetsk, the mob that took over the state security headquarters also attacked the offices of the Industrial Union of the Donbass, a group 49 per cent owned by Donetsk governor Serhiy Taruta. The mob faced no resistance from Ukrainian police in either place.
The angry crowd chanted "Taruta is an enemy of the people" as windows were smashed at the oligarch governor's company and documents, furniture and computers were thrown out of the windows.
At least six people had died in a security operation in Slavyansk on Friday, while more than 40 people were killed in Odessa, where clashes between pro-Ukrainian and pro-Russian protesters culminated in the burning of a trade union building.
Black ribbons were strung to Ukrainian flags hanging from buildings and flowers were laid at makeshift vigils in Odessa to honour dozens of people who were killed in clashes between pro-Ukrainian and pro-Russian protesters on Friday
Kiev also on Saturday released audio tapes of phone calls it purported proved close ties between Vladimir Lukin, a Kremlin envoy sent to negotiate the release of the OSCE German-led military mission taken hostage by separatists last week, and an alleged Russian agent leading the separatists.
Ukraine said the intercepted phone calls proved that Mr Lukin enjoyed a cosy working relationship with Igor Girkin, a senior security official for the separatists in Slavyansk. An arrest warrant has been issued for Mr Girkin, who Kiev say is a Russian citizen and military intelligence agent who created unrest in Crimea before it was annexed by Moscow in March.
In one of the tapes, Mr Lukin is heard asking Mr Girkin: "How warm is the atmosphere, and when can we meet?" In another, the rebel leader is heard telling the Kremlin envoy: "I was given the order to give assistance to you, and not to the European partners."
The OSCE hostages were handed over to Council of Europe secretary-general Thorbjørn Jagland, who travelled with Mr Taruta to meet them at a checkpoint near the regional capital, after their passage out of Slavyansk was delayed by fighting in the nearby city of Kramatorsk. Five Ukrainians taken hostage along with the foreign observers were also freed.
At a press conference in Donetsk on Saturday afternoon, Mr Jagland thanked Mr Lukin, Mr Taruta and other officials for their help in securing the hostages' release.
Russia's President Vladimir Putin moved swiftly to annex Crimea, in the first land grab in Europe since the second world war, and the EU and US are worried over Moscow's intentions elsewhere in Ukraine
Mr Lukin, who also attended the press conference, was asked about the fate of at least two dozen other people, mostly Ukrainian journalists and activists, reporting missing or kidnapped in Slavyansk. The Russian official said that his assignment had been limited to freeing the OSCE hostages. "We solved this problem," he said. "I think there are lots of other problems, but we limited ourselves to this."
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At least two Ukrainians are being held hostage inside Donetsk's regional administration building, which is occupied by separatist militants.
Meanwhile, credit rating agency Standard & Poor's on Saturday warned that a split Ukraine was likely to lead to a default. "If Ukraine loses some of its territorial integrity, it will not likely be able to repay the loans," John Chambers, chairman of the S&P Sovereign Ratings Committee, told Reuters.
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