Russia's parliament on Saturday gave Putin the right to deploy the Russian military in Ukraine.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
President Barack Obama is warning Russia "there will be costs" for any military maneuvers it launches in Ukraine, a move U.S. and Ukrainian officials say they believe to be already underway.
Officials say Obama may retaliate by canceling a trip to Russia this summer for an international summit and could also cut off trade discussions with Moscow. But it's unclear whether those moves will have any impact on Russia's calculus in Ukraine, which is at the center of what many see as a tug of war between East and West.
"Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilizing," Obama declared Friday in a statement from the White House. Such action by Russia would represent a "profound interference" in matters that must be decided by the Ukrainian people, he said.
Hagel said Friday that, while he would not address specific U.S. options, "this could be a very dangerous situation if this continues in a provocative way." Asked about options in a CBS News interview, he said that "we're trying to deal with a diplomatic focus, that's the appropriate, responsible approach."
As Obama prepared to speak late Friday, a spokesman for the Ukrainian border service said eight Russian transport planes had landed with unknown cargo in Ukraine's Crimea region. Serhiy Astakhov told The Associated Press that the Il-76 planes arrived unexpectedly and were given permission to land, one after the other, at Gvardeiskoye air base.
U.S. officials said they also believed Russian personnel had entered Crimea. The State Department urged U.S. citizens to defer nonessential travel plans in Ukraine because of "the potential for instability."
Russian President Vladimir Putin is scheduled to host the Group of Eight economic summit in June in Sochi, the site of the recently completed Winter Olympics. The U.S. is in discussions about the summit with European partners and it is difficult to see how some of those leaders would attend the summit if Russia has forces in Crimea, according to the administration officials. They were not authorized to discuss the situation by name and spoke only on condition of anonymity.
Obama canceled a bilateral meeting with President Vladimir Putin last year after Russia granted asylum to National Security Agency leaker Edward Snowden, though Obama still attended a separate international meeting in Russia.
The administration's warning that trade talks could be halted came as Russian officials were in Washington for economic discussions with Obama advisers.
For the U.S., levying punishments on Russia is complicated by the myriad issues on which the White House needs Moscow's help. Among them: ending the bloodshed in Syria, negotiating a nuclear agreement with Iran and transporting U.S. military troops and equipment out of Afghanistan through Russian supply routes.
At the White House, a somber Obama decried the situation in Ukraine and warned about deeper outside intervention.
"Just days after the world came to Russia for the Olympic Games, that would invite the condemnation of nations around the world," he said. "The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."