Both the Kremlin and the White House admit that U.S. President Obama and Russian President Putin had a tense phone call Monday night.

Putin told Obama that America should "use its capabilities" to prevent the use of "force and bloodshed" while conflicts continue in east Ukraine. 

Obama then told Putin that he should "use his influence" to get pro-Russian separarists to lay down their arms. The White House says Obama is expressing "grave concern" that the Russian government is supporting "armed pro-Russian separatists." Putin denies this.

The White House said Putin intiated the phone call Monday evening as tensions appear to have heightened with CIA Director John Brennan visiting Kiev Monday, and a Russian war plane flying awfully close to a U.S. warship in the Black Sea.

The Obama administration continues to warn Russia that it will respond if they continue displaying acts of aggression against Ukraine. Obama told Putin the U.S. will inflict more "costs" during Monday's phone call.

"I can assure you that Russia's provocations -- further transgressions and provocations will come with a cost. And I'm not here to specify what cost will come from which specific action, but there have already been costs imposed on Russia; there will be further costs imposed on Russia," White House spokesman Jay Carney said.

The White House has accused Russia of supporting the aggressions in Ukraine. On Saturday, the White House blamed the Russian government for the recent violence and chaos in eastern Ukraine.

Putin dismisses these accusations and calls them inaccurate. He denies that Moscow is involved in any of the pro-Russian protests in Ukraine.

On Monday, the pro-Russian separarists refused to vacate occupied government buildings in Ukraine. Current acting Ukranian President Oleksandr Turchynov is asking for a United Nations peacekeeping intervention in the eastern part of the country. 

The U.S. has recently agreed to loan Ukraine $1 billion. That's in addition to the up to $27 billion the country is expected to receive from the International Monetary Found as well as from Europe and the U.S.