US President Barack Obama warned Russian President Vladimir Putin in a call  last night that Moscow would face further costs if its actions in Ukraine persisted, the White House said in a statement.

"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 statement said.

"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," it said.