Obama urges Putin to rein in separatists

The phone call between the two leaders came as pro-Russian activists continued to occupy buildings in eastern towns.

For his part, Mr Putin rejected accusations of Russian interference, calling the reports "unreliable".

EU foreign ministers said on Monday they would expand a list of names targeted by sanctions.

Tension has been steadily rising since Russia annexed the Crimean peninsula, formerly part of Ukraine, last month.

'Grave concern'

The White House said the "frank and direct" conversation between the two presidents was made at Russia's request.

"The president expressed grave concern about Russian government support for the actions of armed, pro-Russian separatists who threaten to undermine and destabilise the government of Ukraine," a statementsaid.

"The president emphasised 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 also threatened Moscow with wider sanctions, saying "the costs Russia already has incurred will increase if those actions persist".