Crimea's regional assembly has declared independence from Ukraine and applied to become part of Russia, a day after a referendum in Crimea overwhelmingly supported joining the Russian Federation.
A delegation of Crimean lawmakers is set to travel to Moscow Monday to discuss additional procedures required to become part of the Federation.
Meanwhile, Ukraine's parliament has approved a partial mobilization of reservists in response to the crisis.
The United States and its European allies are expected to announce sanctions Monday against Russia.
U.S. President Barack Obama told his Russian counterpart Vladimir Putin Sunday that Washington and its "European partners are prepared to impose additional costs" on Moscow for backing the secession referendum in Ukraine's Crimean peninsula.
A White House statement called Sunday's referendum illegal and said it violates Ukraine's constitution. It also said the vote will "never be recognized by the United States and the international community."