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.
"If we allow Kiev to continue to not implement the absolutely necessary political measures with regard to Donbass, the situation could fall out of control with unpredictable consequences," he warned, referring to the disputed zone in eastern Ukraine.
US Ambassador Samantha Power shot back.
"The recent violence is rooted in a combined Russian-separatist assault," she said.
Amid growing alarm over the truce, Power is due to travel to Kiev next week to meet with Ukrainian politicians and get a first-hand account of the situation.
Fighting surged on Wednesday near the village of Maryinka, but clashes subsided in Thursday and Friday in the Donetsk region.
President Petro Poroshenko has warned of a full-blown Russian invasion in the east of the country, accusing Moscow of deploying more than 9,000 troops in support of the separatists.
The Kremlin has accused Kiev of seeking to torpedo the fragile truce ahead of an EU summit in June to discuss whether to extend sanctions against Russia that are to expire in July.
The United States, the EU and Germany all expressed alarm over the renewed violence, with Washington warning Russia it would face new punishment if the crisis escalates.
AFP
