US Secretary of State John Kerry has urged Russian President Vladimir Putin to publicly call on separatists in Ukraine to lay down their arms.
He said Russia would face “greater costs” unless it took action to show that it supported peace in Ukraine.
Earlier, German Chancellor Angela Merkel warned Russia of new sanctions after fresh clashes in eastern Ukraine.
Ukraine’s president has said continued violations might end a truce between the government and pro-Russian rebels.
Chancellor Merkel and French President Francois Hollande spoke to Mr Putin and Ukrainian President Petro Poroshenko for an hour on Wednesday, officials said.