* Six hours of London talks end with deadlock unbroken
* Kerry says Russia to go ahead with referendum in Crimea
* Lavrov says no common vision with the West
By Lesley Wroughton and Dmitry Zhdannikov
LONDON, March 14 (Reuters) - The United States and Russia found no middle ground on the Ukraine crisis on Friday, with the Russians saying they would respect an independence referendum in Crimea and the Americans vowing to impose sanctions if they do.
After six hours of talks with Russian Foreign Minister Sergei Lavrov in London, U.S. Secretary of State John Kerry warned of "an even greater response" if Russia, which shipped more troops and armour into Crimea on Friday, increased tensions further by deploying troops into eastern Ukraine.
The next 48 hours will be key as Crimea, which is part of Ukraine but has an ethnic Russian majority, votes on Sunday on whether to join Russia.
Time is running out for diplomatic efforts to persuade Russian President Vladimir Putin not to annex Crimea, which would unleash a raft of Western sanctions.
"The foreign minister made it clear that President Putin is not prepared to make any decision regarding Ukraine until after the referendum on Sunday," Kerry told reporters, calling the plebiscite a violation of international law and saying it would not be recognised by the international community.
Putin said on Friday the vote was entirely legal.
"I was clear with Foreign Minister Lavrov that the President (Obama) has made clear there will be consequences if Russia does not find a way to change course, and we don't say that as a threat, we say that as a direct consequence of the choices that Russia may or may not choose to make here," Kerry said.
During their meeting at the U.S. ambassador's residence in London, Kerry raised serious concerns about the large number of Russian forces massing on the eastern border with Ukraine, where many ethnic Russians live, and in Crimea.
Speaking afterwards, Lavrov said there was still no common vision with the West over Ukraine and that Russia needed no international structure to help mediate with Kiev.
"We will respect the expression of the will of the Crimean people in the upcoming referendum," Lavrov said. "The Russian Federation does not and cannot have any plans to invade the southeastern regions of Ukraine."
"BACKDOOR ANNEXATION"
A senior U.S. official, speaking on condition of anonymity, said Kerry presented Lavrov with a number of concrete proposals to defuse tensions and address concerns by Moscow over security and protection of minorities within a united Ukraine.
The official said the talks hit a wall early on when it became clear that Lavrov was not authorised by the Kremlin to discuss any proposal that might impact Crimea before the referendum.
"Specifically we shared ideas about greater autonomy in the areas of language, taxation, electing local officials, budgeting, education, health ... a far higher level of autonomy than Crimea has now, with an understanding it would remain a piece of Ukraine," the official said, "But Lavrov did not pick up on any of those because he was not empowered to talk about the future of Crimea at all."
Washington has said it will be ready to move on Monday to impose visa bans and asset freezes, involving Ukrainian and Russian officials, if the referendum goes ahead and it leads to the annexation of Crimea. The European Union and other Western powers have said they will match U.S. sanctions.
"Neither we nor the international community will recognise the results of this referendum and we also remain deeply concerned about the large deployments of Russian forces in Crimea and along the eastern border" Kerry said.
"Right now, given this particular climate, we really need to hear more declarative policy in order to make clear where Russia is proceeding with respect to these troops and exercises."
Kerry said the next steps would depend on a final decision by Putin after the referendum. Ratification of the referendum by Russia's parliament, the Duma, would amount to the "backdoor annexation of Crimea," he said.
The United States has posed several questions to Moscow in a one-page letter that explores whether Moscow would be willing to calm tensions by withdrawing its forces back to barracks and agreeing to international monitors in Crimea.
Ukraine has said it is willing to negotiate with Russia and is prepared to guarantee autonomy for Crimea within Ukraine.
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