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.

A mob of several hundred people led by masked men stormed the state security headquarters in Donetsk chanting pro-Russia slogans on Saturday. They then moved to Industrial Union of the Donbass, the group 49 per cent owned by Donetsk governor Serhiy Taruta and smashed their way into the building. They faced no resistance from Ukrainian police in either place.