The 12 men, among 25 arrested on the same charges, were found dead in their cells at Tanwalbougou, in the east of the country, the local prosecutor announced on May 13.

The local gendarmerie has already launched an investigation into their deaths.

"All the people were executed in a systematic manner," said Aziz Diallo on Monday. Diallo's cousin, 34-year-old Abdoulaye Barry, was one of those who died.

"All the witnesses at the cemetery (where the bodies were initially buried) were of the opinion that they had been executed," he added.

"My cousin was executed with a bullet in the head," said Diallo, who is deputy mayor of the town of Dori, adding that he and the other witnesses had inspected the body at the morgue.

His cousin and his family had fled their home in Yagha province in the north of the country "because of terrorist attacks", he said.

The CISC, a local group campaigning against what it says is the impunity of the security forces, has called for an international inquiry.

CISC general secretary Daouda Diallo said the witnesses had said that all of the bodies had head wounds and that their shrouds were stained with blood.

He described what had happened as "summary executions".

The group is still trying to find out what happened to others among the 40 or so people arrested at the same time.

The security forces in Burkina Faso, as well as local vigilante groups, have been repeatedly accused of abuses over the last three years, particularly against members of the Fulani community.

Some Fulani have joined the jihadist groups in the region who since 2015 have killed more than 850 in Burkina Faso and it is not uncommon to hear local people associate the Fulani community with the jihadists.

Local sources have told AFP that most of the victims were from the Fulani community.