The West African nation has suffered years of conflict with Islamic militants operating in the vast and inhospitable Sahel desert, with thousands of soldiers and civilians killed to date.

The national armies of Niger and its neighbours Mali and Burkina Faso have been accused of war crimes in their response operations, including forced disappearances and extrajudicial killings.

Niger's National Commission on Human Rights was investigating reports by Amnesty International and other rights groups that 102 civilians had gone missing in the western province between March 27 and April 2 after an army operation.

"There have indeed been executions of unarmed civilians and the mission discovered at least 71 bodies in six mass graves," said Abdoulaye Seydou, the president of the Pan-African Network for Peace, Democracy and Development, which participated in the probe, on Friday.

"It is elements of the Defence and Security Forces (FDS) which are responsible for these summary and extrajudicial executions," he added, saying those killed were attacked with bladed weapons and small arms.

But he said the investigation was unable to establish whether senior levels of the military hierarchy were responsible for the deaths.

Jihadist violence resulted in 4,000 deaths in Mali, Niger and Burkina Faso last year, the United Nations has said.

The UN has condemned what it said was a spike in criminal acts by national armies in the Sahel at the beginning of this year, including more than 100 extrajudicial executions by the Malian army between January and March.

Amnesty International reported in June that the armies of Niger, Mali and Burkina Faso had been responsible for nearly 200 disappearances in the space of a few months.