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.
Read MoreThe faces of the Ukrainian conflict
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.