At least 44 suspected members of Boko Haram who were arrested during a recent operation against the group have been found dead in their prison cell in Chad after apparently being poisoned, the country's chief prosecutor has announced.

The prisoners were found dead on Thursday, Youssouf Tom said on television, and 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.

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The dead men were among a group of 58 suspects captured during a major army operation around Lake Chad launched by President Idriss Deby 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," Tom said, referencing the central African country's capital. "On Thursday morning, their jailers told us that 44 prisoners had been found dead in their cell."

"We have buried 40 bodies and sent four bodies to the medical examiner for autopsy," he added.

An investigation was continuing to determine exactly how the prisoners had died, he said.

Accusations of mistreatment

A security source told the AFP news agency 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), has made similar accusations.

Prison officials had "locked the prisoners in a small cell and refusing them food and water for three days because they were accused of belonging to Boko Haram", Ibedou told AFP. "It's horrible what has happened."

The government has denied the allegations.

"There was no ill-treatment," Chad Justice Minister, Djimet Arabi, told AFP by telephone.

"Toxic substances were found in their stomachs. Was it collective suicide or something else? We're still looking for answers," he said, adding that the investigation was still on.

One of the prisoners was transferred to hospital on Thursday, but he was "faring much better" and had rejoined "the other 13 prisoners still alive and who are doing very well", the minister said.

Deadly attack

The government launched the military operation against Boko Haram in response to a devastating attack on Chadian troops on March 23 at a base in Bohoma, in the Lake Chad marshlands, that killed 98 soldiers. It was the largest one-day loss the army has ever suffered.

The operation, which ran from March 31 to April 8, has killed more than 1,000 of the group's fighters and cost the lives of 52 soldiers, according to a Chadian army spokesman.

Since the March 23 attack, Chad's president has warned his allies in the region that Chadian army will no longer take part in operations outside the country.

The force, considered one of the best in the region, has fought Boko Haram in the Lake Chad region as part of the Multinational Joint Task Force (MNJTF) with Nigeria, Cameroon, Niger and Benin since 2015.

Chad is also part of the so-called G5 Sahel force - comprising soldiers from Burkina Faso, Mali, Mauritania and Niger, as well - fighting armed groups in the Sahel. On Friday, French Defence Minister Florence Parly said Chad remained committed to the group.