President Vladimir Putin is on a collision course with the United States and Europe over Ukraine after Russia made clear that it has no intention of easing its grip on Crimea.
Secretary of State John Kerry warned of "consequences" after his counterpart Sergey Lavrov made clear 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.
Earlier, Foreign Secretary William Hague, who met with both men separately, said that the US and the EU could move to sanctions as early as next week it there was no easing of the tensions.
Mr Lavrov, however, was adamant that Russia would respect the will of the Crimean people as expressed in Sunday's referendum vote.
"Everybody understands that Crimea for Russia is something really important, what it means for Russia," he said, speaking through an interpreter. "It means immeasurably more for Russia than the Falklands means for the United Kingdom."
The talks, held at the residence of the US ambassador in Regent's Park, overran by more than two hours, raising hopes that a breakthrough was on the cards.
However, Mr Lavrov finally emerged to tell reporters that there was no "common vision" on a way forward, while Mr Kerry said that he had put forward a series of "constructive ideas", only to be knocked back.
Mr Kerry said that he had reiterated President Barack Obama's warning that there would be "consequences" if the Kremlin did not find a a way to "change course".
"Neither we nor the international community will recognise the results of this referendum," he said. "We believe that a decision to move forward by Russia to ratify the vote officially within the Duma would in fact be a back-door annexation of Crimea."
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 annex," 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 on Sunday had effectively created a "diplomatic deadline" for any further talks to succeed before "other measures" would have to be taken.
"This is a referendum which 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.
"The framework of foreign ministers' contacts were never ceased, so any questions there are could be raised in direct dialogue."
Mr Lavrov insisted Moscow had no plans for a "military intrusion" into the largely pro-Russian eastern Ukraine following clashes in the city of Donetsk, between pro- and anti-Moscow demonstrators.
However Mr Kerry said the Kremlin needed to be clearer that military exercises involving thousands of Russian troops on the Ukrainian border were not intended as a threat to their neighbour.
"All of us would like to see actions not words that support the notion that people are moving in the opposite direction and in fact diminishing their presence," he said.
"I think right now, in this particular climate, given what has been happening, we really need to hear a more declarative policy in order to make clear where Russia is proceeding with respect to these troops and these exercises."