Five Ukrainian servicemen were killed in the worst upsurge of fighting in the east of the country in months, Ukrainian officials said on Thursday.
Presidential aide Yuri Biryukov reported the Ukrainian deaths on his Facebook page and said 39 servicemen had also been injured in battles near the town of Maryinka, which Kiev says the pro-Russian rebels had tried in vain on Wednesday to seize.
The fighting went far beyond regular low-level skirmishing and severely tested a cease-fire brokered in February by the leaders of Ukraine, Russia, Germany and France.
Russia said Kiev was reneging on its cease-fire obligations, while the State Department pointed a finger at Moscow, saying it had a responsibility to lean on the rebels to halt the fighting.
A Ukrainian military spokesman said the situation near Maryinka remained "tense but stable" Thursday morning.
"On three occasions the (separatist) fighters shelled our positions during the night. At the moment there is an operation going on to find sabotage and diversionary groups," he said.
Separately, the Ukrainian military said rebels had fired overnight at government troop positions outside the port city of Mariupol, as well as on villages near Luhansk city in the northern part of rebel-controlled territory.
In Moscow, Russian Foreign Minister Sergei Lavrov, whose country is under economic sanctions over its role in the Ukraine crisis, put the blame for the renewed fighting squarely on Kiev.
"The February Minsk [cease-fire] agreements are constantly under threat because of the actions of the Kiev authorities, trying to walk away from their obligations to foster direct dialogue.”