Ukrainian rebels in Donetsk claim an overnight explosion at an arms factory was probably caused by a government missile.
The rebel stronghold lies in the middle of the Ukrainian industrial heartland and explosions like this have long been feared.
The blast sent a massive orange cloud into the sky and blew out windows of several homes in the surrounding area. No one is reported to have been injured or killed.
However at least nine people are said to have died as a result of continuing heavy shelling around the eastern city.
Intense fighting is still going on around the town of Debaltseve, which lies in a pocket between the two main separatist controlled regions.
Nine Ukrainian troops and seven civilians are reported to have been killed in the past 24 hours.
Deteriorating living conditions are forcing more and more civilians to leave frontline towns.
More than 5,000 civilians, Ukrainian soldiers and pro-separatist fighters have been killed since a separatist rebellion erupted in April.