Burkina Faso's security forces have allegedly executed 31 unarmed detainees in a northern town, Human Rights Watch said, calling for an immediate investigation.

The men, all from the Fulani ethnic group, were allegedly killed just hours after being arrested on April 9 during a government counterterrorism operation, the New York-based rights group said on Monday.

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The incident took place in Djibo, about 200km (120 miles) north of the capital Ouagadougou.

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.

Security forces made "a brutal mockery of a counterterrorism operation, that may amount to a war crime and could fuel further atrocities", said Corinne Dufka, HRW's director for the Sahel region.

HRW interviewed 17 people over the killings, including 12 witnesses to the arrests and later burial of the victims, a statement said.

Local people speculated the ethnic Fulani victims were targeted because of the recent presence of some armed fighters around Djibo.

They said dozens of security 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 pick-up trucks, an armoured vehicle, and motorcycles.

Burkina Faso, one of the world's poorest countries, has battled an armed uprising since 2015. The conflict has provoked attacks on Fulani herders, whom other communities accuse of supporting rebels.

HRW said it sent its findings to the Burkinabe government, but its statement did not include any response from the authorities.