U.S. Secretary of State John Kerry is in London for talks with Russian Foreign Minister Sergei Lavrov on the crisis in Ukraine.
Friday's talks come as the Ukrainian region at the center of the crisis -- Crimea -- prepares for a referendum Sunday that could see it break away from Ukraine and join Russia.
U.S. and European leaders have called the Russia-backed referendum "unconstitutional," and Kerry has warned Moscow of serious consequences if it annexes the Ukrainian peninsula.
But Russian officials say voters in Crimea have a right to self-determination following last month's collapse of the pro-Russian government in Ukraine. And Moscow on Thursday acknowledged deploying thousands more troops and military hardware near the Ukrainian border for maneuvers that will continue for two weeks.
The deployment is in addition to the 20,000 troops Kerry estimates Russia may have in Crimea.