President Barack Obama told Russian President Vladimir Putin in a tense phone call on Monday that Moscow would face further costs for its actions in Ukraine and should use its influence to get separatists in the country to stand down.
Armed pro-Russian separatists seized more buildings in eastern Ukraine earlier in the day, expanding their control after the government failed to follow through on a threatened military crackdown.
In a call that the White House said Moscow requested, Obama told Putin that those forces were threatening to undermine and destabilize the government in Kiev.
"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 a statement.
Obama said Russian troops needed to withdraw from Ukraine's border to defuse tensions and made a point of praising Kiev for its "remarkable restraint" and efforts to unify the country with elections, constitutional reform and proposals to decentralize power to local governments.
"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.
"(He) 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 within Ukraine, and escalatory rhetoric by Kremlin officials."
The Kremlin said Putin told Obama during the call that Russia was not interfering in Ukraine and urged Washington to use its influence to prevent bloodshed.
Earlier, U.S. officials stopped short of announcing a new set of sanctions against Russia but said they were in consultations with European partners about the prospect.