Foreign Secretary William Hague will meet European counterparts tomorrow for talks on Ukraine amid a "dangerous escalation" of the crisis, officials said.
Interim president Oleksandr Turchynov has announced he is sending in the army after pro-Russian forces took over government buildings in the east of the country.
Moscow has denied it is behind the coordinated operation by gangs wearing uniforms without insignia, but Britain said Russia would be assumed to be complicit unless it condemned the actions.
The Foreign Office warned the occupations were a "further dangerous escalation of an already dangerous situation".
In a televised address, president Turchynov said he was launching a "large-scale anti-terrorist operation" to resist attacks and prevent Russian forces moving in.
He said: " The Security Council has made a decision to begin a large-scale anti-terrorist operation with participation of army forces.
"We're not going to allow Russia to repeat the Crimean scenario in Ukraine's east."
It follows u nrest in parts of eastern Ukraine, including Donetsk, where ethnic Russians fear they will suffer at the hands of the West-facing Ukrainian government.
Mr Hague is meeting foreign minsters in Luxembourg where the crisis will be number one on the agenda, officials said.
A Foreign Office spokesman said: " The latest occupations by armed groups of government buildings in towns in eastern Ukraine are a further dangerous escalation of an already dangerous situation.
"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.
"European foreign ministers will meet on Monday to discuss how to respond to the latest events."