The White House says President Barack Obama spoke with Russian President Vladimir Putin by phone for 90 minutes Saturday.
A written statement says the president told Putin sending Russian troops into Ukraine is a clear violation of Ukrainian sovereignty and territorial integrity and a breach of international law.
Obama called on the Russian leader to withdraw his forces to bases in Crimea and to refrain from any interference elsewhere in Ukraine.
The president said Russia's actions would hurt its standing in the international community, echoing a warning he issued on Friday, when the White House threatened costs for Russia's military intervention in Ukraine.
“Indeed the United States will stand with the international community in affirming there will be costs for any military intervention in Ukraine," Obama said.
Russia's upper house of parliament recommended Saturday that President Putin recall the Russian ambassador from Washington.
Many U.S. experts agree that President Obama's options are limited. They say he is not seriously considering using force, although he is said to have talked with advisers about the idea of moving warships to the region.
Ukraine is not a member of NATO, and the Western alliance is not required to come to its defense.
Economic options appear more likely. One possibility is for the U.S. to impose sanctions on Russia's financial institutions.
Tim Brown, senior fellow at globalsecurity.org, says another is to freeze Russian leaders' offshore bank accounts.
"It's almost certain that Putin has a large, or several large, bank accounts of his own, and I would imagine that those are going to get frozen," Brown said. "So when they say there are going to be costs, it's not going to be to the Russian people. It's more along the lines it's going to be the Russian leadership personally."
Saturday's White House statement said the president told Putin the U.S. would suspend its participation in preparatory meetings for the Group of Eight (G8) summit, which is scheduled for June in Sochi. Obama and his aides are reported to have discussed canceling his trip to the summit altogether, or even kicking Russia out of the G8.
Scrapping a possible trade agreement with Moscow may be another alternative.
U.S. leverage is diminished by the need for Russia's help in negotiating a nuclear deal with Iran and working to end Syria's war. Also, the U.S. uses Russian supply routes to move troops and equipment out of Afghanistan.
So far, the possible costs have not been enough to discourage Putin from sending Russian troops to Ukraine.