Washington (AFP) - In some of its sharpest criticism to date, Washington accused Moscow on Friday of "undermining" the global order by supporting rebels in eastern Ukraine.

Germany and France also demanded that a crumbling Ukraine truce be "fully respected," but the words appeared hollow as pro-Russian rebels celebrated a battlefield victory in the strategic town of Debaltseve.

"Russia's continued support of ongoing separatist attacks in violation of the ceasefire in eastern Ukraine is undermining international diplomacy and multilateral institutions -- the foundations of our modern global order," State Department spokeswoman Jen Psaki said.

She insisted that the ceasefire deals reached in Minsk in September and renewed last week were "the basis for a durable resolution in eastern Ukraine."

"We call upon Russia to honor its commitments immediately with decisive action before we see more cities decimated and more lives lost in eastern Ukraine," Psaki said.

The European Union, United States and Ukraine accuse Russia of being behind the hostilities. Moscow denies directly supporting the rebels.

A fresh ceasefire was meant to go into force on Sunday, but Ukraine and the US have pointed to repeated violations, including the seizure of Debaltseve.

"By not abiding by the agreement they signed, by continuing to support and intervene illegally in Ukraine... they're violating international norms and they're violating international law," Psaki said, referring to Russia.

She added that Washington was in contact with the Organization for Security and Cooperation in Europe -- the international body tasked with monitoring the Ukraine ceasefire -- to see what extra help the US could provide.

"Our goal is to ensure that they are well equipped to carry out their task, including monitoring and implementation of the ceasefire, and monitoring the international border between Ukraine and Russia," Psaki said.

The OSCE's mandate runs until March, and it currently has some 500 monitors in the country.

But Psaki renewed US frustration that the rebels have barred access to the OSCE monitors to many areas.

"We'd all love more visibility into what's happening on the ground," she said.