Speaking as the European Union prepared to strengthen its sanctions on Russia for its role in the conflict, UN political chief Jeffrey Feltman yesterday warned the Security Council meeting that the truce was in danger.
"We are either looking at a return to a deepening intractable conflict or a momentary upsurge in parts of the conflict zone. We cannot afford either scenario," he said.
The latest flareup in east Ukraine has left at least 28 dead and sparked fears that the escalation will derail the hard-won ceasefire brokered by France and Germany four months ago.
The economic sanctions, which target Russia's banking and oil sectors, will be extended for six months. They were first imposed after the downing of Malaysia Airlines flight MH17 over rebel-held areas in 2014.
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Meanwhile, the Ukrainian military accused pro-Moscow rebels of targeting its positions around Donetsk, while separatists said Kiev's forces had used Grad rockets against them.
Ukrainian army spokesman Andriy Lysenko said rebel tanks and artillery were "firing virtually without a pause at our positions."
Lysenko said four Ukrainian soldiers had been wounded over the past 24 hours, while separatist commander Eduard Basurin said one rebel had been killed and four wounded.
At the Security Council meeting in New York, the UN official declared: "The ceasefire must be fully respected and the protection of civilians a priority."
At the United Nations, Russia and the United States traded barbs over who was responsible for the latest upsurge in fighting.
Russian Ambassador Vitaly Churkin accused Kiev's forces of targeting civilians in shelling attacks and of failing to fulfill its commitments to reach a political agreement.
"A critical moment has come," said Churkin.