The White House said President Barack Obama urged Russian President Vladimir Putin in a phone call to agree to a peace deal.
"If Russia continues its aggressive actions in Ukraine, including by sending troops, weapons, and financing to support the separatists, the costs for Russia will rise," the White House statement added.
A four-way summit in Minsk aimed at resolving the Ukraine crisis is still very likely to go ahead as scheduled later today despite a number of unresolved issues, French Foreign Minister Laurent Fabius said.
The planned meeting between leaders from Russia, Ukraine, France and Germany comes after rockets killed more than 10 civilians and soldiers in Ukrainian government-held territory yesterday.
Video footage recorded from Kramatorsk, deep inside Ukrainian-government controlled territory, shows rockets landing on civilian areas.
Video footage appears to show the moment a shell strikes a community soup kitchen in eastern Ukraine.
A mother in eastern Ukraine buries her son - yet another casualty in a civil war that is killing civilians with appalling randomness.