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 is already underway.
Officials say Obama may retaliate by canceling a trip to Russia this year for an international summit and could cut off trade discussions with Moscow. But it's unclear whether those moves will have any impact on Russia's strategy on Ukraine, which is at the center of what many see as tensions between East and West.
"Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilizing," Obama declared Friday. Such action by Russia would represent a "profound interference" in matters that must be decided by the Ukrainian people, he said.
Defense Secretary Chuck Hagel said 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."
A spokesman for the Ukrainian border service said Friday that 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, a peninsula on the Black Sea with an ethnic Russian majority and a Russian naval base. 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 Putin last year after Russia granted asylum to National Security Agency leaker Edward Snowden, though Obama still attended a separate international meeting in Russia.
For the U.S., levying punishments on Russia is complicated by the various 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.
A somber Obama decried the situation in Ukraine and warned about deeper outside intervention.
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," he said.
Political turmoil in Ukraine has pushed President Viktor Yanukovych from office. Yanukovych held a news conference in Russia on Friday in which he said he was not asking Moscow for military assistance and called military action "unacceptable."
Yanukovych, who still regards himself the president, also vowed to "keep fighting for the future of Ukraine" and blamed the U.S. and the West for encouraging the rebellion that forced him to flee last weekend.
One catalyst for the massive demonstrations that led to Yanukovych's ouster was his rejection of a partnership agreement with the European Union in favor of historical ties with Moscow. That EU agreement would have paved the way for Ukraine's greater integration with the West, including potential affiliation with NATO, something to which Russia strongly objects.
Secretary of State John Kerry and other senior U.S. officials have tried without success to dispel widespread sentiment in Russia that the United States and Europe are trying to pry Ukraine out from under Russian influence.
But in Moscow, Russian officials have been accusing the U.S. and its allies of meddling, fomenting anti-Russia sentiment and actively encouraging Ukraine's Western aspirations at the expense of its historical connections.
There was no known contact Friday between Obama and Putin, who last spoke a week ago.
Kerry did call Russian Foreign Minister Sergey Lavrov for the second time in two days to press the Kremlin to keep its promise to respect Ukraine's sovereignty and territorial integrity.
Lavrov repeated Putin's pledge to do just that, while pointing out that Russia has broad interests in Ukraine, Kerry said.
Kerry reiterated the U.S. view that Russian military intervention in Ukraine following the ouster of the country's Russia-backed leader would run counter to Russia's self-professed opposition to such operations in other countries, such as Libya and Syria.