Following heavy shelling in what had been a Ukraine-controlled city, the central government's force is retreating from Debaltseve, a key railroad and transportation hub. Ukraine says it has now withdrawn 80 percent of its armed forces from the city.

"I can say now that the Ukrainian armed forces and the National Guard completed an operation on the planned and organized withdrawal of some units from Debaltseve this morning," Ukraine's President Petro Poroshenko said, according to the Interfax news agency in Ukraine.

"Some 80 percent of the units have already been pulled out," he said before leaving to visit eastern Ukraine Wednesday. He added that two more columns of troops will be withdrawn from Debaltseve.

Poroshenko is seeking a "tough reaction" from international leaders who brokered the recent cease-fire with Russian-backed separatists. A spokesman for German Chancellor Angela Merkel, who was instrumental in the peace talks, says the rebels are committing "a massive violation" of the temporary peace.

A conference call is scheduled for later today, in which Ukraine, Russia, France and Germany will discuss possible reactions, according to a French official.

From Moscow, NPR's Corey Flintoff reports:

"A cease-fire was officially supposed to have taken effect on Sunday, but relentless shelling kept up around Debaltseve, a railroad hub defended by hundreds of Ukrainian troops.

"The troops have been effectively surrounded by Russian-backed militias for days.

"Observers from the Organization for Security and Cooperation in Europe say the separatist militias are preventing them from entering the area to monitor the cease fire."

Corey says the separatists have insisted that the peace agreement doesn't apply to Debaltseve.

"One reason they're so determined to take that town," he says, "is that it was part of a Ukrainian-controlled pocket that pokes deep into the separatist front lines."

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