The United States believes a hard-won Ukraine ceasefire accord is "imperfect" but still much better than having nothing at all to help resolve the bloody conflict, a senior US official said Friday.

US ambassador to NATO Douglas Lute said the agreements also offered "a glimmer of diplomatic hope" on ending a war between Kiev and pro-Russian rebels in eastern Ukraine which has cost more than 6,000 lives.

"Implementation of Minsk has been mixed and uneven but it is better than no Minsk," Lute told reporters ahead of NATO foreign ministers meeting next month where Ukraine will again be a top agenda item.

A first peace accord agreed in September in the Belarus capital Minsk between Ukraine and the rebels and backed by the European Union and Russia, failed to halt the fighting.

France and Germany then brokered a second Minsk agreement in February which appears to be working better although breaches are reported daily as Ukraine and the rebels swap charges of bad faith.

Lute said the ceasefire at least provided some "rules of the road" when there had been none before, even if the rebels were doing less on meeting the terms of the accord than the Ukraine government.

"There is a diplomatic promise in place now. This is worthwhile and even offers a glimmer of hope bit it is not a bright light," he added.

US Defense Secretary Ashton Carter warned Wednesday that rebel forces appeared to be preparing for fresh attacks just as they sat down for fresh talks with Kiev.

Ukraine's representative, former president Leonid Kuchma, said after a meeting in Minsk Wednesday that it had been a "serious step towards the political resolution" of the conflict but the shooting had to stop completely.