The United States warned Friday that President Barack Obama and other European leaders could snub the G8 summit in Sochi if they judge Russia is usurping Ukraine's sovereignty.
Obama said that he was deeply concerned at reports that Russian forces had landed in Crimea, following claims by an official in Kiev that an "invasion" was under way.
Senior officials said the president decided to make a surprise appearance in the White House briefing room after Washington concluded that there was evidence that Moscow's forces were indeed active in Ukraine.
Heavily armed troops in uniforms with no national insignia were seen around government buildings and the airport in the Crimean city of Simferopol, as Ukrainian officials accused Russia of "naked aggression."
The crisis has mushroomed from a political and economic one following the ouster of a pro-Kremlin government in Kiev into a geopolitical showdown after claims by Ukrainian officials that around 2,000 Russian troops had landed in the Crimea.
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," Obama said.
A military official told AFP after the president's statement that "it looks like they've moved several hundred troops there."
The official said that the Kremlin had given no advance warning of the movement.
Obama recognized that Russia had interests and cultural and economic ties with Ukraine, and had a military facility in Crimea, a territory ceded to the Soviet republic of Ukraine by the Soviet Union in 1954.
But he said any violation of Ukraine's sovereignty and territorial integrity would be "deeply destabilizing."
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," Obama said.
Another senior official later suggested those costs could include a decision by Obama and top European leaders to skip the summit of G8 industrialized nations in the Black Sea Olympic resort of Sochi in June.
Other possible trade and commercial concessions that the Kremlin has been seeking as recently as this week could also be at risk, the official said.
Moscow would also squander any global goodwill spurred by the Winter Games, which ended last weekend, US officials argued.
TRYING TO AVOID COLD WAR SCENARIO
Beyond that though, the extent of US leverage on Russia over Ukraine remained unclear.
There is no suggestion of US or allied military action, and the route to a UN Security Council resolution would be blocked by Russia as a permanent member.
Washington is also trying to avoid a Cold War-type scenario with Moscow, and needs Putin's support on a string of issues, including talks between world powers and Iran over its nuclear program, the effort to destroy Syria's chemical weapons and on withdrawing its troops from Afghanistan.
Obama warned that a Russian military intervention in the post-Soviet state would "represent a profound interference in matters that must be determined by the Ukrainian people."
He said that Vice President Joe Biden had spoken Friday with the new prime minister of Ukraine Arseniy Yatsenyuk to offer Washington's support for his country's democratic future.
Washington would continue to communicate directly with the Russian government on the crisis, Obama added.
Earlier, Washington's envoy to the United Nations Samantha Power called for urgent international mediation in Crimea and for Russia to withdraw forces it was building up in the region.
"The United States calls on Russia to pull back the military forces that are being built up in the region, to stand down and to allow the Ukrainian people the opportunity to pursue their own government, create their own destiny and to do so freely without intimidation or fear," Power said after an emergency Security Council meeting.
"The United States calls for an urgent, international mediation mission to the Crimea to begin to de-escalate the situation and facilitate productive and peaceful dialogue among all Ukrainian parties."
Also on Friday, US Secretary of State John Kerry spoke by telephone with his Russian counterpart Sergei Lavrov for the fourth time in seven days.