At least ten soldiers died in an ambush in southwestern Niger close to the border with Mali by a group described as "armed terrorists."
Government sources say the death toll from Friday's attack is expected to rise as 16 people are still missing and others were wounded.
The troops were on patrol in the north of Banibangou department when they faced an assault by a rebels described as a jihadist group.
Last week armed groups stormed a camp, in the Tahoua area, housing refugees from neighbouring Mali. Nine people were killed in the attack.
According to the United Nations, over 61,000 Malian refugees currently shelter in Tahoua and Tillaberi.
On Saturday evening, France "condemned in the strongest possible terms the attack that left ten dead".
The French Foreign Ministry expressed "its full solidarity with the Nigerian authorities and people" and stands by them "in the fight against terrorism."
France is supporting Niger in its Almahaou operation to secure the border with Mali, with 250 soldiers in a "combat partnership".
Niger is also one of the options for the redeployment of French special forces, driven out of neighbouring Burkina by the ruling junta.
After the departure of French soldiers from Mali last year and a scheduled pullout shortly from Burkina Faso, France will only station 3,000 troops in the Sahel region including Niger and Chad, where jihadist groups roam.
All the countries involved are former French colonies.