U.S. Secretary of State arrived in London on Friday to hold last-ditch talks with Russian Foreign Minister Sergey Lavrov ahead of Sunday's proposed referendum in Crimea, which, if passed, would allow Russia to annex the region from Ukraine. Kerry hopes to delay the vote with threats of sanctions, though it's expected to take place as scheduled.

"If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here [in Washington] with respect to the options that are available to us," Kerry said before arriving in London, according to the BBC.

The referendum will allow Crimean citizens to vote to secede from Ukraine and formally join the Russian Federation.

British Prime Minister David Cameron met with Kerry before his meeting with Lavrov. "We want to see Ukrainians and the Russians talking to each other and if they don't then there are going to have to be consequences," Cameron said after the meeting.

On Thursday, Russia told the UN it did "not want war," though earlier in the week President Putin ordered thousands of troops to perform drills along the Ukrainian border.

"It is clearly political coercion, at a minimum," a Western official told the New York Times. The unnamed official also said large numbers of Russian citizens were being bused to cities in eastern Ukraine to undermine the new Ukrainian government.

"We are very concerned," a State Department official told the Times. "This is the second time inside of a month that Russia has chosen to mass large amounts of force on short notice without much transparency around the eastern borders of Ukraine. It certainly creates an environment of intimidation."

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