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 his Russian Foreign Minister Sergei Lavrov in London, U.S. Secretary of State John Kerry warned of "an even greater response" if Russia sent troops into eastern Ukraine.
"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.
"We believe the referendum is contrary to the constitution of Ukraine, contrary to international law, is in violation of that law, and is illegitimate.
"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.
U.S. officials said the door was open for more talks and Kerry said he would keep in touch with Lavrov in the coming days.
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.
During their meeting at the U.S. ambassador's residence in London, Kerry raised concerns over Russian troop movements.
"Neither we nor the international community will recognize 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" he added.
"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.
Speaking after the talks, Lavrov said Russia had no plans to invade southeastern Ukraine but made clear that the Kremlin would respect the result of the referendum.
He 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 can not have any plans to invade the southeastern regions of Ukraine."
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.
Crimea has an ethnic Russian majority and many in the province of two million people favor rule from Moscow.
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