WASHINGTON, April 14 -- U.S. President Barack Obama on Monday urged his Russian counterpart Vladimir Putin to convince pro-Russian armed groups to leave the buildings they seized in Ukraine, and withdraw Russian troops from Ukraine's border to defuse tensions.

In a phone call with Putin at Moscow's request, Obama expressed "grave concern" about Russia's support for the actions of pro-Russian armed groups, the White House said in a statement.

"The President emphasized that all irregular forces in the country need to lay down their arms, and he urged President Putin to use his influence with these armed, pro-Russian groups to convince them to depart the buildings they have seized," the statement said.

The U.S. president also reiterated the importance of Russia withdrawing its troops from Ukraine's border in order to defuse tensions, the statement said.

Meanwhile, Obama lauded Ukrainian government for acting with "remarkable restraint" in responding to Russian officials' rhetoric and Kiev's efforts to unify the country by holding free and fair presidential elections on May 25.

Citing Russia's "growing political and economic isolation" as a result of its recent actions in Ukraine, Obama warned that the costs that Russia has already incurred after Crimea, a former Ukraine's autonomous republic, joined Russia last month "will increase if these actions persist."

Russia, however, has denied alleged involvement in Ukraine's domestic affairs

"We don't interfere in Ukraine's internal affairs; this contradicts our interests," Lavrov told reporters Monday following his talks with visiting Sudanese counterpart Ali Karti.

Lavrov said that those who encourage Ukrainian authorities to use force to suppress protests in the country's eastern regions should be held to account for the consequences.