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.
At least one person was killed when a shell hit a bus station in the centre of rebel-controlled Donetsk city, east Ukraine, a witness told Reuters.
The body of a man could be seen behind the wheel of a minibus after a shell fell through the roof of the station, burning up the vehicle and another beside it, he said.
Emergency services at the scene said another person had died in hospital following the attack, while another two were seriously wounded. There was no immediate comment from rebel authorities or the Kiev-controlled regional administration.
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.