As Ukrainian leaders accused Russia of carrying out an armed invasion in the Crimea region, U.S. President Barack Obama warned Moscow not to intervene militarily.
"We are deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine. Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilizing," said U.S. President Barack Obama.
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," Obama added.
He said any military intervention by Russian forces would represent a "profound interference" in the Ukrainian people’s right to determine their own future.
Earlier Friday, armed men described by Ukrainian officials as Russian troops seized the parliament building in Simferopol, Crimea's capital, while others patrolled outside the autonomous region's main airport.
Armored personnel carriers with Russian markings appeared on roads on the peninsula, a predominantly Russian-speaking area with stronger historical ties to Moscow than to Ukraine’s central government in Kyiv.
The Russian navy also blockaded a Ukrainian coast guard station. Russia has numerous military facilities in Crimea, including the headquarters of its Black Sea Fleet, in Sevastopol.
Invasion and occupation
Earlier, Ukraine's parliament speaker, Oleksandr Turchynov, who is now the country's acting president, accused Russia of a military invasion and occupation.
"The Russian Federation has begun an overtly aggressive campaign against our country. Under the pretense of military training they have bought military force into the Autonomous Republic of Crimea, and have not only captured the parliament and Cabinet of ministers, they are also attempting to take control of civilian strategic points such as communications," said Turchynov.
Moscow denied its forces had entered Crimea. Russian officials said any troop movements there are in keeping with Moscow's existing agreements with Ukraine.
Russian military vehicles regularly transverse the peninsula, but the size and scope of Friday’s activity was larger than usual.
Ukraine's Defense Ministry said it had information that unknown "radical forces" are planning attempts to disarm Ukrainian military units in Crimea early Saturday and warned against such action.