Russia announced new military operations Thursday near the Ukraine border, prompting acting Ukraine President Oleksandr Turchynov to warn that those forces appear "ready to intervene at any time."
Russian media say the exercises will take place over the next two weeks, as the United States and Western Europe face off with Moscow over the fate of Ukraine's Russian-speaking Crimean peninsula.
Moscow, which sent thousands of military personnel into Ukraine earlier this month, is backing a referendum Sunday that was called to decide whether Crimea should secede from Ukraine and possibly join the Russian Federation.
U.S. and European leaders accuse Russia of interfering in Ukraine's internal affairs. They say the vote is illegal and have threatened Moscow with economic and diplomatic sanctions if Crimea leaves.
In Moscow Thursday, the Russian Defense Ministry acknowledged major operations with armored and airborne forces near its border with eastern Ukraine. The ministry said the maneuvers involve artillery units, assault helicopters and at least 10,000 soldiers.
Earlier this week, NATO announced its own deployment of fighter jets and surveillance aircraft in European territories bordering Russia.
In Washington Thursday, Secretary of State John Kerry testified at a U.S. Senate hearing, reiterating the U.S. position that the referendum violates the constitution of Ukraine, as well as international law and the charter of the United Nations. He estimated Russia may have 20,000 troops in Crimea.
"There is no justification, no legality to this referendum that is taking place. It violates international law, it violates the U.N. charter, it violates the constitution of Ukraine, and I don't think anybody can believe that a hastily put together, rushed referendum, taking place under the imprint of 20,000 plus troops and all that has happened, without debate, without opportunity, is a genuine referendum."
Kerry and his Russian counterpart, Sergei Lavrov, are set to discuss Ukraine Friday at a face-to-face meeting in London.
U.S. President Barack Obama warned Russia again Wednesday that the West will "apply costs" to Moscow if it continues to interfere in Ukrainian affairs.
Speaking at the White House alongside interim Ukrainian Prime Minister Arseniy Yatsenyuk, Mr. Obama said Washington "completely rejects" Crimea's planned referendum Sunday. He said the vote, "patched together in a few weeks," is a violation of international law.
U.S. Republican Senator John McCain was leading a bipartisan delegation to Kyiv Thursday. A spokesman on Wednesday described the visit as a show of congressional support for the interim government, "and for the Ukrainian people's aspirations for freedom, democracy and territorial integrity."