President Obama spoke with Russian President Vladimir Putin on Monday about the escalating crisis in Ukraine, urging him to use his influence to convince pro-Russian forces to stand down and leave the buildings they are occupying in eastern Ukraine.

“The president expressed grave concern about Russian government support for the actions of armed, pro-Russian separatists who threaten to undermine and destabilize the government of Ukraine,” the White House said in a read-out of the phone call released Monday night.

The White House said Obama spoke to Putin at Moscow's request and that he reiterated the importance of Russia dispersing troops amassed at the eastern border of Ukraine.

If Putin doesn't cooperate, Obama told him Russia would be further isolated economically in the international community and will incur increasing costs if it continues to try to intimidate the Ukrainian government.

The read-out of the phone call did not include Putin's response to Obama's demands. Russian officials have denied that the militants occupying building in eastern Ukrainian towns are acting on orders from Moscow, but U.S. officials say the sieges have all the telltale signs of the actions taken in Crimea just a few weeks ago.

During the conversation with Putin, Obama highlighted the importance of Thursday's meeting in Geneva of U.S., Russian, Ukrainian and European Union officials aimed at trying to negotiate an end to the crisis in Ukraine.

Obama told Putin that while he continues to believe that a diplomatic solution is still possible, “it cannot succeed in an environment of Russian military intimidation on Ukraine’s borders, armed provocation within Ukraine, and escalatory rhetoric by Kremlin officials.”

After meeting in Luxembourg on Monday, European Union foreign ministers agreed to impose additional sanctions on Russian officials, and French Foreign Minister Laurent Fabius threatened that the group's leaders could decide to increase the pressure on Russia next week if a solution is not reached.