The men had entered the tank at the Oued Ghir prison outside Bejaia in order to clean it, state media reported earlier.

Their bodies were transferred to hospital to undergo an autopsy.

"One prisoner and seven guards died of suffocation in a septic tank inside the Oued Ghir prison, at the edge of the facility," Zeghmati said, quoted by the official APS news agency.

"The circumstances of the accident will be determined after a preliminary investigation," according to Zeghmati, who visited the site on Wednesday afternoon with Interior Minister Kamel Beldjoud.

Bejaia civil defence chief Nourredine Fedi said the men had died after inhaling hydrogen sulphide, a toxic gas, adding that civil defence personnel had cleaned the tank before retrieving the bodies.

Opened in 2010, the Oued Ghir prison has a capacity of 1,000 inmates.

Authorities at the time said the facility "meets modern standards" for detention facilities.