All irregular forces in Ukraine need to lay down their arms: Obama

He further urged Putin to use his influence with these armed, pro-Russian groups

Expressing grave concern over Russia's support for separatists groups in Ukraine, US President Barack Obama has told his Russian counterpart Vladimir Putin that all irregular forces in Ukraine need to lay down their arms.

"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 White House said in readout of the call yesterday.

The leaders talk comes ahead of the meeting between the Secretary of State John Kerry and his Russian counterpart Sergei Lavrov in Europe later this week.

Reiterating the importance of Russia withdrawing its troops from Ukraine's border in order to defuse tensions, Obama noted that despite the rhetoric from Russian officials, the government of Ukraine has acted with remarkable restraint.

He praised the Ukrainian government's efforts to unify the country by holding free and fair presidential elections on May 25, pursuing inclusive constitutional reform and proposing concrete steps toward the decentralization of power.

"The President noted Russia's growing political and economic isolation as a result of its actions in Ukraine and made clear that the costs Russia already has incurred will increase if those actions persist," the White House said.

Obama noted the upcoming contact group meeting in Geneva and said 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 with in Ukraine and escalatory rhetoric by Kremlin officials, it added.