Pro-Russian activists have attacked another official building in eastUkraine, ignoring a deadline to leave or face eviction by Ukrainian forces.
A crowd stormed a police station in the town of Horlivka, near Donetsk, taking control of the building.
Ukraine's interim president hit out at "aggression" from Russia, but signalled support for a national referendum.
Olexander Turchynov said Kiev was "not against" a vote on the future of the country, a key demand from protesters.
Mr Turchynov also said Ukraine was preparing an "anti-terrorist operation" against gunmen occupying government buildings in Sloviansk and a number of other towns and cities.
His office said that he had suggested the UN could assist in any such operation. This would be highly unlikely as Russia has a veto on the Security Council, which would have to authorise any such action.
Correspondents say people in eastern Ukraine are anxiously waiting to see if Mr Turchynov carries through on his threat to use the army against the pro-Russian groups.