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.
A senior State Department official says the U.S. is "very concerned" about the deployment, which adds to the 20,000 troops Kerry estimates Russia may have in Crimea.
The official said this is the second time in a month "that Russia has chosen to mass large amounts of force on short notice without much transparency around the eastern borders of Ukraine."
He said Kerry will ask Lavrov about the move, calling it "destabilizing" and saying it "creates an environment of intimidation."
The U.S. secretary of state is meeting with British Foreign Minister William Hague and Prime Minister David Cameron ahead of his talks with the Russian foreign minister.
On Thursday, Ukraine's interim prime minister, Arseniy Yatsenyuk, confronted Russian Ambassador Vitaly Churkin at the U.N. Security Council, asking him bluntly whether "Russians want war."
Churkin replied that neither his government nor his people want war. He said, "I am convinced that Ukrainians don't want this either. We don't want any further exacerbation of the situation."
But U.S. and European leaders accuse Russia of interfering in Ukraine's internal affairs and have threatened Moscow with economic and diplomatic sanctions if Crimea leaves Ukraine.
Washington's U.N. ambassador, Samantha Power, addressing the Security Council Thursday, called for "a climate of restraint" and for "peaceful dialogue to replace [Russian] coercion."
She accused Russia of pursuing a course of military action "from the outset" of the crisis, which erupted in full in late February when pro-Russian President Viktor Yanukovych -- facing weeks of anti-government protests -- fled the country.
Earlier this week, as tensions mounted over the standoff, NATO announced its own deployment of fighter jets and surveillance aircraft in European territories bordering Russia.