President Vladimir Putin is on a collision course with the United States and Europe over Ukraine after Russia it made clear it has no intention of easing its grip on Crimea.
US Secretary of State John Kerry warned of "consequences" after his counterpart Sergey Lavrov made it plain Moscow would respect the outcome of a referendum in Crimea this weekend on breaking away from Ukraine and joining Russia.
After more than five hours of face-to-face talks in London ended without agreement, Mr Kerry said any move by the Russian parliament to ratify a breakaway vote would amount to a "back door annexation" of the peninsula.
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UK Foreign Secretary William Hague, who met the men separately, described the failure of the talks to achieve a breakthrough as "deeply disappointing" and called for a "firm and united" response from EU foreign ministers when they meet in Brussels on Monday.
He said the time had come for "tougher restrictive measures to be adopted as previously agreed by EU nations".
"The door remains open for diplomacy and dialogue and we urge the Russian government to take action to lower tensions and enter into direct talks with the Ukrainian government," he said.
"But we are clear that the flagrant violation of the sovereignty and territorial integrity of Ukraine cannot go unchallenged, and we stand with the Ukrainian people in their right to choose their future free of intimidation and interference."
The talks, held at the residence of the US ambassador in Regent's Park, London, overran by more than two hours, raising hopes a breakthrough was on the cards.
However, Mr Lavrov emerged to say there was no "common vision" on a way forward, while Mr Kerry said he had put forward a series of "constructive ideas" only to have them rejected. Mr Kerry said he had reiterated President Barack Obama's warning that there would be "consequences" if the Kremlin did not find a way to "change course".
He added: "If the wrong choices are made, then there will be no choice but to respond appropriately because of the gravity of this breach of international standard, this breach of international law." He sought, however, to keep open the door to further diplomacy, saying there were "many options" for Mr Putin to respect the result of the referendum without moving to annexation.
"You can respect the vote by making sure that their autonomy is increased, that their needs that have prompted that vote are properly respected, without necessarily made the decision to annexe," he said.
"Until he has made his decision, those options are still on the table and alive and we hope he will make a different set of choices."
Mr Hague said the decision by the Crimean parliament to stage the vote tomorrow had effectively created a "diplomatic deadline" for any further talks to succeed before "other measures" would have to be taken.
"This is a referendum that doesn't meet any international norm or standards; it's taking place without a campaign, without an electoral roll, without the political leaders of the country being able to visit, in the presence of many thousands of troops from a foreign country," he said.
Mr Lavrov however rejected calls for the creation of an international contact group to resolve the crisis, insisting that it was a matter for Ukraine and Russia.
"We do not need an international structure to look into Ukrainian-Russian relationship. The relationships have never been ceased," he said.