The US military's top-ranking officer on Tuesday added to a growing clamor for Washington to arm Ukraine in its battle with pro-Russian forces, saying he favors doing so through NATO.

It was the first time General Martin Dempsey had spoken out in support of arming Ukraine's army against the separatists and follows similar comments by the Pentagon's new chief, Ashton Carter, and the director of national intelligence, James Clapper.

"I think we should absolutely consider lethal aid and it ought to be in the context of NATO allies because (Russian President Vladimir) Putin's ultimate objective is to fracture NATO," Dempsey told the Senate Armed Services Committee.

President Barack Obama is weighing a possible move to provide weapons to Kiev but some NATO members -- including France and Germany -- are opposed to arming Ukraine over fears it could further escalate the conflict in eastern Ukraine.

Western powers on Tuesday suggested Moscow could face more punitive sanctions if Russia violates a February 12 truce between Kiev government troops and the pro-Russian rebels.

Both sides in the conflict claim to be upholding the ceasefire and pulling their artillery back from the frontline.

The leaders of Ukraine, Russia, France and Germany also have agreed to send observers to flashpoints in eastern Ukraine to monitor the truce, officials in Kiev said.

Leaders in Ukraine fear the port city of Mariupol will be the next target of separatists because it could offer a land bridge to the Crimea peninsula -- annexed by Russia a year ago.

The conflict, which has dragged on for 11 months, has left 6,000 dead, according to a UN tally.