The U.S. administration has evidence that Russia intends to supply heavier multiple rocket launchers to separatists in eastern Ukraine, U.S. State Department spokesperson Marie Harf has said.

"We have new evidence that the Russians intend to deliver heavier and more powerful multiple rocket launchers to the separatist forces in Ukraine," she said at a briefing on Thursday.

"We have evidence that Russia is firing artillery from within Russia to attack Ukrainian military positions," Harf said.