They died on Saturday when their vehicle hit an explosive device in the northeast, just days after three others were killed in similar fashion.

The deaths brought to 50 the number of French soldiers killed in the West African nation since France first intervened in 2013 to help drive back jihadist forces.

The Group to Support Islam and Muslims (GSIM) said it "detonated an explosive device" as the vehicle was passing, "bringing the toll to five in less than a week", in a statement released by its propaganda platform Al-Zallaqa late on Monday.

The attack killed corporal Loic Risser, 24, and sergeant Yvonne Huynh, 33, the mother of a young child and the first female soldier killed since the French operation began in the Sahel region.

The GSIM also denied responsibility for an attack on two villages in western Niger on Saturday which killed 100 people -- the biggest single massacre of civilians in the Sahel's eight-year-old Islamist insurgency.

"This attack, whoever carried it out, is not different from the massacres of the French occupiers and criminal militias, the GSIM said, adding that its "jihad" has not turned against the people, and vowed reprisals.

The group appeared to be referring to the Islamic State in the Greater Sahara (EIGS), its great rival in the region and with whom it has clashed violently in recent months.

One year ago, French President Emmanuel Macron designated the EIGS as the number one enemy in the Sahel region.

Since then the GSIM has grown in strength, and in November Barkhane force commander Marc Conruyt named the group the "most dangerous" in the region.

France's Barkhane operation counts 5,100 troops spread across the arid Sahel region and has been fighting jihadist groups alongside soldiers from Mauritania, Chad, Mali, Burkina Faso and Niger, who together make up the G5 Sahel group.

The GSIM statement was authenticated by SITE Intelligence, which monitors jihadist activities worldwide.