British Foreign Secretary William Hague on Saturday condemned the Russian parliament's decision to authorise military action in Ukraine and said he had summoned the Russian ambassador to register his concerns.
"This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine. We condemn any act of aggression against Ukraine," Hague said in a statement issued by the Foreign Office.
The foreign secretary will visit Kiev on Sunday for talks with the interim Ukrainian government on the rapidly escalating situation in the Crimea.
The Kremlin said that President Vladimir Putin had not yet taken a decision on sending troops to Ukraine, after he was given the green light from parliament.
Washington has warned that such a deployment would result in "costs" for Moscow.
Hague said: "I am deeply concerned at the escalation of tensions in Ukraine, and the decision of the Russian parliament to authorise military action on Ukrainian soil against the wishes of the Ukrainian government."
He said he spoke to Russian Foreign Minister Sergei Lavrov "to urge steps to calm this dangerous situation", while the Foreign Office also summoned the Russian ambassador to London "to register our deep concerns".
In a telephone call with German Foreign Minister Frank-Walter Steinmeier, Hague also agreed "on the need for international diplomatic action to address the crisis".
EU foreign ministers will hold a new round of crisis talks on Ukraine in Brussels on Monday, when Hague said he would raise the issue of a further asset freeze.
During his visit to Kiev, Hague said he would discuss how Britain can help the Ukrainian government recover "improperly acquired assets".
"The EU must agree urgently an asset freezing regime to target those suspected of laundering the proceeds of corruption," Hague said, without naming names.
"On my instructions, the British embassy in Kiev has told the Ukrainian government that we stand ready to provide Ukraine with technical advice on asset recovery."
Hague spoke to acting Ukrainian president Oleksandr Turchynov on Friday, offering Britain's support and urging him to take steps to unify the crisis-hit country.