A senior administration official says Secretary of State John Kerry will travel to the Ukrainian capital of Kiev Tuesday to meet with government officials.
Kerry's trip comes as the U.S. and other Western nations weigh a response to Russia's military advance into Ukraine. Kerry has raised the possibility of issuing economic sanctions and visa bans, freezing Russian assets and levying trade and investment penalties.
The official says the U.S. now believes that Russia has complete operational control of the Crimea peninsula, a pro-Russian area of Ukraine. The official says the U.S. believes there are more than 6,000 Russian air and naval forces in the region.
The official was not authorized to publicly discuss the situation and insisted on anonymity.
Secretary of State John Kerry calls Russia's military incursion into Ukraine 'an incredible act of aggression" and says President Vladimir Putin has made "a stunning, willful" choice to invade another country.
Kerry says Russia should respect the democratic process through which the Ukrainian people ousted their pro-Russian president and assembled a new government.
Kerry is raising the possibility of boycotting the June meeting of the Group of Eight leading industrialized countries in Sochi, Russia.
He's also discussing visa bans, asset freezes, and trade and investment penalties.
Kerry said he spoke with foreign ministers for G-8 and other nations on Saturday, and says everyone is prepared 'to go to the hilt" to isolate Russia.
Kerry was interviewed Sunday on CBS, ABC and NBC news shows.
President Barack Obama is calling on Russia to de-escalate tensions in Ukraine by pulling its forces back to bases in the country's Crimean peninsula and to refrain from any interference elsewhere in Ukraine.
The White House says Obama delivered that message to Russian President Vladimir Putin during a 90-minute telephone call on Saturday afternoon.
A statement from the White House says the U.S. condemns Russia's military reach into Ukrainian territory.
Russia says Putin emphasized that real threats exist to the life and health of Russian citizens and that Russia has the right to protect its interests and the Russian-speaking people who live in that part of Ukraine.
Russia's parliament on Saturday gave Putin the right to deploy the Russian military in Ukraine.