WASHINGTON (AFP) - The United States cautiously welcomed a brokered ceasefire in the 10-month war between Ukraine and pro-Moscow rebels Thursday, but insisted the deal must be implemented and honoured.

"The agreement represents a potentially significant step toward a peaceful resolution of the conflict and the restoration of Ukraine's sovereignty," the White House said in a statement.

The deal was negotiated between leaders of Germany, France, Russia and Ukraine after marathon talks that ended early Thursday.

The ceasefire is due to take effect on Feb 15.

It would create a 50km buffer zone and require the withdrawal of troops and heavy artillery from the front line, under the eye of monitors from the OSCE.

The White House said Russia must now "end its support for the separatists and withdraw its soldiers and military equipment from eastern Ukraine."

"The true test of today's accord will be in its full and unambiguous implementation." That would include "the durable end of hostilities and the restoration of Ukrainian control over its border with Russia," the White House said.

The agreement comes amid a fresh wave of hostilities on the ground that could call the deal into question even before the ink is dry.

The White House said it was "particularly concerned about the escalation of fighting today, which is inconsistent with the spirit of the accord."

In some corners of Congress, the agreement received a lukewarm welcome.

"It's got more holes than a piece of Swiss cheese," Republican Senator John McCain told AFP.

"While this is going on, Russian tanks and equipment are pouring into eastern Ukraine. I see no provision for enforcement of border control between Ukraine and Russia."

McCain suggested it could be a repeat of a deal signed last year which went largely ignored.

"I am unfortunately rather confident it'll go the way of the Minsk agreement," he said.

Secretary of State John Kerry said there was a "long road ahead before achieving peace."