US Secretary of State John Kerry is meeting Russian counterpart Sergei Lavrov in London in a last-ditch attempt to resolve the crisis in Ukraine's Crimea peninsula.
Kerry arrived in Britain on Friday to talk to Lavrov for the third time this month, two days before a vote that could result in Crimea joining the Russian Federation.
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A senior State Department official told the Reuters news agency that Kerry would seek assurances that Moscow would not annex Crimea and that it would work with Ukraine to neutralise tensions in the peninsula, where pro-Kremlin supporters and Russian troops have steadily increased their presence during the past few weeks.
The official, who briefed reporters before the trip, said: "If the Russians choose not to take that course, if Putin chooses not to avail himself with that opportunity, then there will be costs. We have made clear there is a preferred way to deal with this, which is to begin de-escalating."
A difficult situation
Lavrov made some brief remarks before going into the talks, which were held at the US ambassador's residence in London.
He said: "This is a difficult situation. Many events have happened and a lot of time has been lost. Now we have to see what can be done."
US and EU leaders have in the past weeks engaged in frantic bouts of diplomacy to defuse a crisis that has severely strained relations between Russia and the West.
|Tensions in Ukraine continue as Crimea vote nears
Kerry has warned Moscow that the US and the EU will impose sanctions on Russia as early as Monday if the referendum is held. He and his European counterparts have repeatedly denounced Russia's intervention in Crimea, adding that Sunday's result is irrelevant.
However, Moscow has welcomed the decision of pro-Kremlin politicians in Crimea to align themselves with the Russian Federation and has hinted that it may extend its hand in Ukraine.