The 12 men, among 25 arrested for "suspected acts of terrorism," 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.

But Human Rights Watch said the Burkina Faso authorities "should credibly and independently investigate alleged extrajudicial executions of 12 men" detained by gendarmes on May 11.

"Suspects winding up dead hours after being taken into custody during counterterrorism operations is a strong indication of foul play," Corinne Dufka, Sahel director at Human Rights Watch, said in a statement.

"Killing detainees in the name of security is both unlawful and counterproductive. Those found responsible for these deaths in detention should be fully and fairly prosecuted."

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 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 people 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.

The Burkina Faso government has said two investigations have been launched, one judicial and the other administrative.