Fears of serious violence in Ukraine are growing as the country's president announced a "large-scale anti-terrorist operation" to resist attacks by armed pro-Russian forces, after a gun battle in which one man was killed.
Foreign Secretary William Hague will meet European counterparts in Luxembourg today for talks on Ukraine amid a "dangerous escalation" of the crisis.
In a televised address, President Oleksandr Turchynov said authorities in Kiev will use the army in order to prevent Russian forces from moving in, as they did in Crimea. He pledged an amnesty to anyone laying down arms by this morning.
"The Security Council has made a decision to begin a large-scale anti-terrorist operation with participation of army forces," he said. "We're not going to allow Russia to repeat the Crimean scenario in Ukraine's east."
Moscow has denied it is behind the operation by gangs wearing uniforms without insignia, but Britain said Russia would be assumed to be complicit unless it condemned the actions.
"The latest occupations by armed groups of government buildings in towns in eastern Ukraine are a further dangerous escalation of an already dangerous situation," a Foreign Office spokesman said.
"Assumptions that Russia is complicit are inevitable as long as Moscow does not publicly distance itself from these latest lawless actions. Russia must desist from steps which destabilise Ukraine and undermine the possibility of Contact Group talks.
Ukrainian special forces exchanged gunfire with a pro-Russia militia in an eastern city yesterday morning, with at least one security officer killed and five others wounded.
It was the first reported gun battle in eastern Ukraine, where armed pro-Russia men have seized a number of government buildings in recent days.
Mr Turchynov said a Security Service captain wa s killed and two colonels wounded in a gun battle outside Slovyansk, where the police station and the Security Service office were seized a day earlier.
Ethnic Russians in Ukraine's east widely fear that the new pro-western Ukrainian government will suppress them.