Intense fighting erupted around east Ukraine's main rebel-held city on Sunday that wrecked a power station and briefly trapped nearly 400 coal miners in one of Europe's largest pits.
Repeated rounds of rocket and mortar fire echoed across snow-covered Donetsk on Sunday evening despite a formal truce in the nine-month war.
Witnesses said the shelling -- particularly heavy on the northern outskirts city where a disputed airport has been under attack for months -- had reached levels last seen at the height of the conflict last summer.
It was not immediately clear what provoked the latest escalation of violence or who had launched the first attack.
But it threatens to derail Ukrainian President Petro Poroshenko's efforts to arrange direct peace talks with Russia's Vladimir Putin at a Thursday meeting in Kazakhstan that would also be attended by the leaders of Germany and France.
A Ukrainian military spokesman in Kiev said the insurgents had targeted federal positions 41 times by Sunday evening.
The spokesman stressed that most of the rebel rocket and artillery strikes occurred around the disputed Donetsk airport that a skeleton crew of government forces has been holding on to since the end of May.
One was reported killed in overnight violence but there was no immediate information about other casualties later in the day.
Local officials said the civilians had also been killed in various incidents across the mostly Russian-speaking east of the separatist region overnight.
Rebels officials said one of the shells fired by Kiev's forces on Sunday afternoon hit a district electricity substation that feeds the city's vast Zasyadko coal mine.
Some 331 miners had been working in the shaft when it went dark, a spokesman for the local coal miners' union told AFP.
The inccident affected the pit's ventilation system and halted lifts and elevators that miners use to return to the ground.
"The power went back up at 3:31 pm and they started pulling up the miners in groups," Independent Ukrainian Miners Union chairman Mykhailo Volynets told AFP.
Volynets said everyone had been safely evacuated within five hours of the power cut.
Zasyadko employs 10,000 people when fully operational and has been beset by problems in the past.
A 2007 disaster at the site claimed the lives of more than 100 people and remains post-Soviet Ukraine's worst industrial accident.
There was no official comment about the incident from either pro-Ukrainian officials in the region or the military.
Merkel presses Putin
One of Europe's deadliest conflicts in decades has claimed more than 4,700 lives since breaking out in April following the ouster in Kiev of a Russian-backed president.
The warring sides agreed to a Russian-backed truce on September 5 that was meant to give the separatist regions limited self-rule within a united Ukraine.
But the terms did not suit some rebel leaders and the more radical field commanders.
The deal's 12 points were never full implemented and fighting has since resulted in 1,300 more deaths.
But Poroshenko's efforts to arrange direct international peace talks have been met with some caution in Europe.
German Chancellor Angela Merkel told both Putin and Poroshenko in separate calls late Saturday that a mini-summit also involving French President Francois Hollande would only make sense if it resulted in "concrete progress".
German government spokesman Steffen Seibert said Merkel specifically told Putin that Russia needed to put more pressure on the rebels to respect the terms of the truce.
The Kremlin denies Ukrainian and Western charges that it had backed the uprising in a bid to cement its influence over the former Soviet republic in the wake of its decision last year to sign a landmark EU pact.
The four nations' foreign ministers will meet in Berlin on Monday to make what Poroshenko hopes are final preparations for Astana.