WASHINGTON -- U.S. officials said President Barack Obama may cancel plans to attend an international summit in Russia this year and could halt discussions on deepening trade ties with Moscow, raising specific possible consequences if Russia intervenes in Ukraine. Obama bluntly warned of unspecified "costs" for Russia.

"Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilizing," Obama said Friday. Such action by Russia would represent a "profound interference" in matters that must be decided by the Ukrainian people, he said.

While the president spoke only of "reports" of military movements inside Ukraine, the officials said the U.S does believe that Russia is intervening.

Separately, Defence Secretary Chuck Hagel said he would not address specific U.S. options, "but 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, a spokesman for the Ukrainian border service said eight Russian transport planes had landed with unknown cargo in the largely Russian-friendly 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.

The State Department urged U.S. citizens to defer non-essential travel plans to Ukraine because of "the potential for instability."

In Washington, it was unclear whether the administration's threats might have any impact. Obama cancelled 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.

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.

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 many issues on which the White House needs Moscow's help. Among them are 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.

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.

Any Russian military incursion in Crimea would dramatically raise the stakes in Ukraine, which is at the centre of what many see as a tug of war between East and West.

One of the catalysts for massive demonstrations that led to Yanukovych's ouster was his rejection of a partnership agreement with the European Union in favour 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.

Their argument, though, seems to be falling on deaf ears in Moscow, where Russian officials have been accusing the U.S. and its allies of meddling, fomenting anti-Russia sentiment and actively encouraging Kyiv'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. Kerry said he warned Moscow against military moves in Crimea that could further inflame tensions.

Lavrov repeated Putin's pledge to do just that while also pointing out that Russia has broad interests in Ukraine, Kerry said.

Kerry reiterated the U.S. view that Russian military intervention in Ukraine would run counter to Russia's self-professed opposition to such operations in other countries, such as Libya and Syria.

 

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Associated Press writers Julie Pace, Matthew Lee and Darlene Superville in Washington contributed to this report.