The authorities have previously faced accusations of extrajudicial killings in the war against a jihadist insurgency that they have been waging since 2015.

The dead, who were men mostly from the Fulani and Peul ethnic groups -- communities targeted by the insurgents for recruitment -- were buried by residents of the northern town of Djibo between April and May.

"So many of the dead were blindfolded, had their hands tied up... and were shot in the head," one of the town's community leaders told Human Rights Watch (HRW) in the report released Wednesday.

"Mass graves containing at least 180 bodies have been discovered in recent months, and available evidence suggests the involvement of government security forces in extrajudicial mass killings," the international watchdog said.

HRW's Sahel director Corinne Dufka called for an urgent investigation into the deaths.

"Existing information points toward government security forces, so it's critical to have impartial investigations, evidence properly gathered, and families informed about what happened to their loved ones," she said in the report.

In response to the allegations, the government said it would open an inquiry.

Officials said these executions may have been committed by armed groups who used the uniforms and munitions of the army when they attack, according to HRW.