A military tribunal prosecutor, who opened the investigation at the request of the defence ministry, visited the site at Gossi in the north of the country on April 23.

An official statement said he would "bring to light all the facts" of the case and keep the public fully informed.

On April 21, two days after French forces handed the base back to Mali, they released a video said to show Russian mercenaries burying bodies nearby to falsely accuse the departing troops.

In the video, filmed with a drone, Caucasian soldiers appear to be covering bodies with sand near the Gossi base.

France officially handed control of Gossi to Mali last Tuesday as part of a withdrawal announced in February.

The next day the Malian army denied any role in burying the bodies.

"The state of advanced putrefaction of the bodies indicates that this mass grave existed well before the handover (of the base). Consequently, the responsibility for this act can in no way be attributed to the FAMa", or Malian armed forces, a statement said.

France and the United States have accused Russian Wagner mercenaries of deploying in Mali as Paris winds down its almost decade-long military operation in the Sahel country.

Bamako's military-dominated government says the Russians are just military instructors.

Anti-French sentiment has grown in West Africa, where French forces have operated since 2013 to stem jihadist insurgencies.

France's pull-out from Mali comes with swathes of territory still under the control of rebels linked to Al-Qaeda and the Islamic State group.