A group of 44 suspected members of Boko Haram, arrested during a recent operation against the jihadist group, have been found dead in their prison cell, apparently poisoned, Chad's chief prosecutor announced on Saturday.

A fighter with the Syrian Democratic Forces fires a mortar shell toward a part of Baghouz where remaining Islamic State (IS) group fighters are holding out in their last position, in the countryside of the eastern Syrian province of Deir Ezzor on Monday. Photo: AFP

Speaking on national television, Youssouf Tom said the 44 prisoners had been found dead in their cell on Thursday.

An autopsy carried out on four of the dead prisoners revealed traces of a lethal substance that had caused heart attacks in some of the victims and severe asphyxiation in the others, he said.

The dead men were among a group of 58 suspects captured during a major army operation around Lake Chad launched by President Idriss Deby Itno at the end of March.

"Following the fighting around Lake Chad, 58 members of Boko Haram had been taken prisoner and sent to Ndjamena for the purposes of the investigation," said Tom.

"On Thursday morning, their jailers told us that 44 prisoners had been found dead in their cell," Tom said, adding that he had attended the scene.

"We have buried 40 bodies and sent four bodies to the medical examiner for autopsy." An investigation was ongoing to determine exactly how the prisoners had died, he said.

A security source, speaking on condition of anonymity told AFP that "the 58 prisoners were placed in a single cell and were given nothing to eat or drink for two days." 

Mahamat Nour Ahmed Ibedou, secretary general of the Chadian Convention for the Protection of Human Rights (CTDDH), made similar accusations.