Reuters – Five Ukrainian servicemen were killed in the worst upsurge of fighting in the east of the country in months and the situation remains tense, Ukrainian officials said on Thursday, as pro-Russian rebels also updated their own death toll.

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, in which both sides used heavy weapons, went far beyond regular low-level skirmishing and severely tested a ceasefire brokered in February by the leaders of Ukraine, Russia, Germany and France.

The rebels have blamed Kiev's forces for the clashes and on Thursday revised their own estimate of casualties in the past day to 19, comprising 15 civilians and four fighters. On Wednesday they had said 15 people in total had been killed.

Russia said on Thursday Kiev was reneging on its ceasefire obligations, while the U.S. State Department pointed a finger at Moscow, saying it had a responsibility to lean on the rebels to halt the fighting.

Concerns over the military escalation in Ukraine caused Russia's rouble to weaken 1.2 percent against the dollar in early trade on Thursday following a 3 percent fall on Wednesday.

A Ukrainian military spokesman said the situation near Maryinka remained "tense but stable" on 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 south-eastern 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 Western economic sanctions over its role in the Ukraine crisis, put the blame for the renewed fighting squarely on Kiev.

"The February Minsk (ceasefire) agreements are constantly under threat because of the actions of the Kiev authorities, trying to walk away from their obligations to foster direct dialogue with Donbass," Lavrov told a security meeting, referring to the rebel-held region in east Ukraine.

On Wednesday, Ukraine's defense ministry said the separatists, using tanks, artillery and up to 1,000 troops, had launched a large-scale offensive to take government positions near Maryinka.

Ukrainian forces held onto their positions during nearly 12 hours of fighting and are still in control of the town and its surrounding area, the defense ministry said.