President Barack Obama warned Russia on Friday that military intervention in Ukraine would lead to "costs".
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," he told reporters.
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," Obama said in the White House briefing room.
Facing yet another confrontation with Putin after butting heads with him over Syria, Obama said any violation of Ukraine's sovereignty and territorial integrity would be "deeply destabilizing."
Obama did not spell out what he meant by Russian military intervention or "costs." .