Burkina Faso's security forces allegedly killed 31 unarmed detainees in the northern town of Djibo, Human Rights Watch said on Monday, calling for an immediate investigation.
The men, all from the Fulani ethnic group, were allegedly killed hours after being arrested on April 9 during a government counterterrorism operation, the New York-based group said.
The incident, which took place in Djibo, about 200 kilometres (120 miles) north of the capital Ouagadougou, made "a brutal mockery of a counterterrorism operation that may amount to a war crime and could fuel further atrocities", said HRW's Corinne Dufka.
The defence ministry said in a statement it was aware of the claims and had asked its military justice department to open an investigation.
HRW interviewed 17 people over the killings, including 12 witnesses to the arrests and later burial of the victims, the statement said.
The rights group noted that armed Islamist groups have largely recruited from the Fulani community, and have attacked security forces as well as civilians, notably in the north of Burkina Faso.
It said it had documented "the killing of several hundred men by government security forces for their alleged support of these groups" since 2017.
Local people speculated that the army had targeted the Fulani because of the recent presence of some armed Islamists around Djibo.
They said scores of security force personnel were involved in the operation, which lasted several hours, HRW said.
The victims were taken away in a convoy of about 10 military vehicles including pickup trucks, an armoured car and motorcycles, it said.
After hearing gunfire, local people found the bodies of 31 men, last seen in the custody of the security forces, said HRW. Several had had their eyes or hands bound.
None of the witnesses saw any of the arrested men with a firearm.
Burkina Faso, one of the world's poorest countries, has battled a jihadist insurgency since 2015.
The conflict has provoked attacks on Fulani herders whom other communities accuse of supporting militants.