France killed the leader of so-called Islamic State in the Greater Sahara because the group attacked French aid workers, African civilians and US troops, French officials said Thursday.

They called him "enemy No. 1" in protracted anti-terrorism efforts in the Sahel.

French President Emmanuel Macron announced the death of Adnan Abu Walid al-Sahrawi overnight.

According to Mr Macron's office, al-Sahrawi personally ordered the killing of six French aid workers and their Nigerien colleagues last year, and his group was behind a 2017 attack that killed US and Niger military personnel.

He was killed in a strike by France's Barkhane military operation "a few weeks ago", but authorities waited to be sure of his identity before making the announcement, French defence minister Florence Parly told RFI radio on Thursday.

She did not disclose details of the operation or where al-Sahrawi was killed, though the group is active along the border between Mali and Niger.

"He was at the origin of massacres and terror," French foreign minister Jean-Yves Le Drian said on France-Info radio.

He urged African governments to fill the void and seize ground taken by the IS extremists.

Rumours of the militant leader's death had circulated for weeks in Mali, though authorities in the region had not confirmed it.

Al-Sahrawi had claimed responsibility for a 2017 attack in Niger that killed four US military personnel and four people with Niger's military.

His group has also abducted foreigners in the Sahel and is believed to still be holding American Jeffery Woodke, who was abducted from his home in Niger in 2016.

The extremist leader was born in the disputed territory of Western Sahara and later joined the Polisario Front.

After spending time in Algeria, he made his way to northern Mali where he became an important figure in the group known as MUJAO that controlled the major northern town of Gao in 2012.

A French-led military operation the following year ousted Islamic extremists from power in Gao and other northern cities, though those elements later regrouped and again carried out attacks.

The Malian group MUJAO was loyal to the regional al-Qaida affiliate. But in 2015, al-Sahrawi released an audio message pledging allegiance to the IS group in Iraq and Syria.

France, the region's former colonial power, recently announced that it would be reducing its military presence in the region, with plans to withdraw 2,000 troops by early next year.