Authorities had spent two days clearing bodies off the streets of the northeastern university city of Maiduguri, which bore the brunt of the violence.

A government spokesperson, Usman Chiroma, said: "Our evacuation team has finished removing all dead bodies from the streets of the city. Families are not forthcoming in claiming the dead bodies. Therefore, the government decided to bury them in mass graves.

Extremist

"It is difficult for them to do so (claim the bodies), because their dead relations were members of the Boko Haram (sect) that waged war against the government. They just don't want to be associated with them," said Chiroma.

Although the police and military declined to give a figure for the number of bodies involved, ThisDay newspaper put it at about 700.

Police and witnesses said more than 600 were killed during five days of violence between security forces and sect members in four northern states - Bauchi, Kano, Yobe and Borno.

But the government, whose forces routed die-hards of the Boko Haram extremist group and killed their leader Mohammed Yusuf on Friday, has yet to release an official death toll.

Cholera

The unrest began  in the Bauchi state last Sunday when Yusuf's followers attacked a police station. Violence later spread to three other states in Nigeria's Muslim north.

The fighting was fiercest in Maiduguri, Borno State's capital, as the military bombarded the headquarters of Boko Haram, killing about 200 poorly armed militants as well as their 39-year-old leader.

An official of the International Committee of the Red Cross had on Saturday raised concerns about a possible disease outbreak in the city.

"We are really worried about a possible outbreak of diseases like cholera due to the presence of decomposing corpses on the streets of Maiduguri, which constitutes a serious health risk.

Amnesty International, Human Rights Watch (HRW) and local rights groups have called for an investigation into Yusuf's killing while he was in the custody of security forces.