As fears persist that eastern Ukraine is about to fall back into full-scale war, a leader of the international monitoring group is urgently calling for resumed negotiations between the warring sides.

The last round of talks under the auspices of the Organization for Security and Cooperation in Europe ended without visible results on Tuesday, and an especially fierce battle broke out in eastern Ukraine the next day. The clash between Ukrainian forces and Russia-backed separatist rebels was the worst fighting seen since a cease-fire began to take hold in March.

"We urgently call on the sides to return to the negotiations table and resolve all the questions," OSCE monitoring mission deputy head Alexander Hug told the United Nations Security Council on Friday, Russian news reports said.