The State Department is warning U.S. citizens to put off all nonessential travel to Ukraine, and particularly Crimea.
The department cited "the potential for instability" following the ouster of Viktor Yanukovych as president and the establishment of a new government.
Groups have staged protests, set up roadblocks and occupied government buildings in several cities throughout Ukraine since November 2013. The State Department warned all U.S. citizens in Ukraine, and those considering travel there to be aware of the possibility of violence.
President Barack Obama spoke of "reports" of military movements inside Ukraine, and officials said the U.S does believe that Russia is intervening.
Ukrainian officials say armed men described as Russian troops took control of key airports in Crimea on Friday and Russian transport planes flew into the strategic region.