Foreign Secretary William Hague has said Russia's unwillingness to de-escalate the crisis in Ukraine has made it a "formidably difficult task" for a diplomatic solution to be reached.
Mr Hague said the US-Russia talks being held by John Kerry and Sergei Lavrov in London would work towards a "breakthrough", adding "every effort will be made to find a diplomatic way forward" over the next few hours in the capital.
But he added that it was "realistic" to prepare action should Sunday's internationally unsolicited referendum in Crimea go ahead with a meeting of EU foreign ministers set for Monday.
John Kerry warned of "consequences" after his Russian counterpart made clear Moscow would respect the outcome of Crimea's referendum.
Foreign ministers meeting in Brussels will decide on Monday on sanctions for Russia if there is no de-escalation in Ukraine.
Crimea's new information minister has told ITV News that the planned referendum is no different to the situation in Scotland and Catalonia.