Militants killed 71 soldiers in an ambush on a remote military camp in Niger near the border with Mali, an army spokesman said on Wednesday.
It was the deadliest raid against the Nigerien military in living memory.
Several hundred militants attacked a base in the western Niger town of Inates over a period of three hours on Tuesday evening, army spokesman Col Boubacar Hassan said.
No group has confirmed responsibility for the killings, but fighters linked to ISIS and Al Qaeda operate in the area and have carried out similar attacks across West Africa's Sahel region this year.
Security forces in Niger have struggled to stop the spread of extremist groups since more weapons arrived in the region after the fall of Muammar Qaddafi in 2011.
ISIS’s West African branch killed nearly 50 Nigerien soldiers in two attacks in the same area in May and July.
The news of Tuesday's attack comes just after the government of Niger has requested a three-month extension to its state of emergency, which it declared two years ago.
"The combat was of a rare violence, combining artillery shells and the use of kamikaze vehicles by the enemy," Col Boubacar Hassan said.
He said 12 soldiers were wounded and others were missing, while a "significant number" of militants were also killed.
Two security sources said 30 soldiers were still missing.
President Mahamadou Issoufou arrived in Niger on Wednesday evening after cutting short a visit to Egypt, his office said in a tweet.
The attack comes at the end of a year of intense violence in Inates, a cattle herding community near the banks of the Niger River, 200 kilometres north of the capital Niamey. Militants killed two village chiefs this year, local sources say.
Since July, hundreds of people have fled the area for Niamey or other nearby towns, the sources said, leaving their cattle and houses untended and unguarded.
Security has deteriorated this year across the Sahel, a semi-arid strip of land beneath the Sahara, amid terrorist attacks and deadly ethnic reprisals between rival farming and herding communities.
The region has been in crisis since 2012, when ethnic Tuareg rebels and other militants seized the northern two thirds of Mali, forcing France to bring troops to the country the following year.
But the militants have since regrouped and expanded their range of influence.
The rising body count this year has inflamed popular anger against regional governments and former coloniser France, which has 4,500 troops across the Sahel.
Frustrated by mounting anti-French sentiment, French President Emmanuel Macron, has invited five West African leaders to a meeting next week.
There he plans to ask them to say whether they want French troops to remain in their countries.
