Rockets fired by Ukrainian military units hit a high-security prison outside Donetsk late Sunday night, killing one, severely injuring three and prompting over 100 potentially dangerous inmates to escape, a Donetsk city official said.

Donetsk City council spokesman Maxim Rovinsky said a riot broke out after the rocket hit. Some of the 106 prisoners who escaped were jailed for murder, rape and other violent crimes, he said. City officials said some prisoners had returned, but did not specify how many. Prison officials say 70 are missing and 30 returned.

Prisoners escaped when a power station providing power to the prison was knocked out by rocket fire. Rovinsky says rockets fired overnight also hit 10 homes, shops and other buildings. Artillery and rocket bombardments have knocked out power for some 20,000 Ukrainians in Donetsk, the namesake city of the east’s largest self-declared separatist republic.

Ukrainian forces have surrounded the separatist stronghold and are making their most intense offensive against separatists yet in their campaign to retake the embattled east. The campaign has prompted 400,000 Donetsk residents – nearly half the population – to flee the city. Business and economic activity in the city has all but shut down, leaving those trapped there without essentials.

The United Nations say over 1,500 people have been killed and 4,300 wounded, including civilians, in the fighting since the Ukrainian government began an offensive against armed separatists groups in the east in April. Separatists took up arms after pro-Russian President Viktor Yanukovych was ousted and a pro-European government took power. Two separatist “states” declared their independence in April.

Both armed militants and Ukrainian government forces accuse each other of indiscriminate violence against civilians in the east. Both also point to each other over the downing of Malaysian Airlines flight MH17, which is suspected to have been shot down by sophisticated military hardware available to both sides. Rebel forces have pleaded for direct help from Russia, but it has refused. NATO strongly accuses Russia of arming and supplying separatists, including with the Buk missile launcher allegedly used to shoot down Malaysia Airlines Flight MH17 over the war-torn region last month.