U.S. President Barack Obama criticized Russia in a telephone call with President Vladimir Putin on Monday, saying its actions were not conducive to a diplomatic solution.
"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.
Moscow put the onus for peace on Washington. "President Putin called on Barack Obama to do his utmost to use the opportunities that the United States has to prevent the use of force, and bloodshed,'' the Kremlin said in a statement.
NATO states have sent troops, aircraft and ships to eastern Europe to reassure nervous post-communist alliance members, including the Baltic states - which were once Soviet republics - as well as Romania and Bulgaria.
A Russian fighter aircraft made repeated low-altitude, close-range passes near a U.S. ship in the Black Sea over the weekend, the Pentagon said, condemning the action at a time of heightened U.S.-Russian tensions.
The White House has warned Russia it would face further costs over its actions in Ukraine, but it made clear that the United States was not considering lethal aid for Ukraine.