More than 1,800 inmates have escaped from a prison in Nigeria after it was attacked by gunmen, officials say, BBC reported.

The attackers reportedly entered the prison yard in the southeastern town of Owerri by using explosives to blast the administrative block.

One inmate was killed and another was slightly injured during the attack.

Six prisoners have reportedly returned, while 35 refused to escape.

Police have accused the banned separatist group, the Indigenous People of Biafra, of carrying out the attack. It has reportedly denied involvement.

The Nigerian Correctional Service has confirmed that 1,844 inmates escaped from the jail in Imo state.

Heavily armed men stormed the Owerri Custodial Centre, after arriving in pickup trucks and buses, the service said.

A police spokesperson said the attackers carried rocket-propelled grenades, machine guns, explosives and rifles.