Donetsk prison escape Ukraine
Inmates stand at a door of a high-security prison after shelling in DonetskGetty

More than 100 prisoners have escaped from a high-security prison after it was hit by shelling in the beseiged rebel stronghold of Donetsk, authorities in eastern Ukraine have confirmed.

Donetsk city council spokesman Maxim Rovinsky said a rocket had hit and killed at least one inmate and left three others severely wounded. The strike sparked a riot, during which 106 prisoners escaped, including some jailed for murder, rape and robbery.

The institution is a "strict regime" prison for dangerous criminals, located on the city's western outskirts.

Rockets struck a sub-station which provided the building with electricity, allowing prisoners to escape afer the facility's alarm system was disabled.

"Extremely dangerous prisoners are now free. It is hard to know the extent of threat this poses to the city, which is flooded with weapons," Rovinsky told Associated Press.

Officials from Ukraine's state penitentiary service said later on Monday that 34 prisoners had since returned to the jail, but it has not been possible to verify that claim.

In the past week, Ukrainian government forces have intensified their military operations and surrounded Donetsk, the largest city held by pro-Russian separatists in eastern Ukraine. Thousands have fled the city as rocket fire increases and the civilian death toll from shelling mounts.

Both Ukrainian government forces and the pro-Russian rebels who want independence for their eastern region have deployed heavy and often imprecise weapons.

Rovinsky said an estimated 10 homes, shops and garages were hit overnight by rockets. He added that around 400,000 have fled the city, which had a pre-war population of one million.

Another 20,000 have no electricity in Donetsk, while shops have closed and basic supplies are dwindling.

The Ukrainians army's strategy has focused on encircling Donetsk and nearby rebel towns and breaking off road links with other separatist towns and villages further east, closer to the Russian border.

Many of those in Russian-speaking eastern Ukraine distrust the new central government in Kiev, which came to power after the February ousting of former President Viktor Yanukovych, whose power base was in eastern Ukraine.

At the weekend, Sky News reported that rebel leader Aleksandr Zakharchenko said the city was surrounded by government troops and that pro-Russian forces were willing to accept a ceasefire.

Fighting erupted a month after Russia annexed Ukraine's peninsula of Crimea in March.