Twenty-nine soldiers were killed in western Niger in an attack by suspected extremist, the Defence Ministry said on Monday night.
The soldiers were attacked using “improvised explosive devices and kamikaze vehicles by more than a hundred terrorists”, the ministry said.
Two soldiers were seriously wounded and “several dozen terrorists” were also killed.
Authorities declared a three-day national mourning period for the soldiers.
The attack took place near the border with Mali, during military operations aimed at “neutralising the threat” posed by ISIS in the area.
“Communications from the terrorists, who were forced to withdraw, have been intercepted,” the ministry said.
It said the attackers “benefited from outside expertise”.
An extremist insurgency has plagued Africa's Sahel region for more than a decade, breaking out in northern Mali in 2012 before spreading to neighbouring Niger and Burkina Faso in 2015.
The “three-borders” area between Niger, Mali and Burkina Faso is often the scene of attacks by militants affiliated with ISIS and Al Qaeda.
The violence has triggered military takeovers in all three countries, with Niger the latest to fall to a coup on July 26 that ousted Mohamed Bazoum, its democratically elected president.
Twelve Nigerien soldiers died last week when their convoy came under attack from militants near the northern town of Kandadji, Gen Salifou Mody, Niger's Defence Minister, said.
In August, at least 17 Nigerien soldiers were killed and 20 wounded in an attack by suspected extremists near the border between Niger and Burkina Faso.