Almost 500 miners are trapped underground after a shell hit an electric substation near the site in war-torn east Ukraine.
Emergency services were alerted when Donetsk's Zasyadko coal mine, one of eastern Europe's largest, lost all power in the blast.
It comes just over a fortnight since a similar incident briefly trapped 300 miners on January 11.
A Donetsk region emergencies ministry official said: "When the power went out, there were 496 miners underground. Right now, we are making preparations to bring them to safety."
Another ministry official was quoted by a pro-rebel website as saying the lives of the miners are not in danger and that the first evacuation had already begun.
An unnamed separatist "defence ministry" official in Donetsk said 110 miners had been brought back to the ground in the first minutes after the power outage.
The war between pro-Russian militants and government forces in eastern Ukraine has claimed nearly 5,100 lives since it began nine months ago.