The White House on Friday said the U.S. has warned Russia that any intervention in Ukraine would be a “grave mistake,” urging Moscow to respect its neighbor's sovereignty.

White House press secretary Jay Carney said the administration is watching to see “whether or not Russia is doing anything that might be crossing the line in any way.”

“We strongly support Ukraine's territorial integrity and sovereignty, and we expect other nations to do the same,” he added. “Reports of Russian intervention in Crimea are of deep concern to the United States.

Reports on Friday suggested that Russian troops were policing airports in Crimea, a region of Ukraine with strong Russian ties.

Carney called those reports “inconsistent” with Russia’s claims that it will respect Ukraine’s territorial integrity.

“We call on Russia to respect its international obligations made under the U.N. charter to refrain from the threat or use of force against the territorial integrity or political independence of any state,” he said. “We urge all parties to avoid steps that could be misinterpreted or lead to a miscalculation at this delicate time.”

Carney would not specify what Russian actions would invite a response, saying only that the White House was watching the situation.

Fears have grown that Russia may seek to undermine the interim government in Kiev after Ukraine’s former president Viktor Yanukovych, who was backed by Moscow, was removed from power.

Yanukovych fled the capital after months of protests by groups opposed to his push for closer ties with Russia erupted into violence. Security forces launched a brutal crackdown on protesters that ended after a European Union-backed peace deal installed an interim caretaker government and set the stage for new elections.

The new Ukrainian parliament voted to remove Yanukovych from office after he left Kiev. But the former president insists he is still the legitimate leader.

The White House said Yanukovych had “abdicated” his office and urged Russia to respect the new government.

Russia, though, announced new military exercises close to the Ukrainian border, sparking worries that Moscow will seek to reassert influence over its neighbor.