Suspected Islamist militants killed 25 soldiers and wounded six others in an attack on an army post in west Niger near its border with Mali on Thursday, the government said in a statement.
It was not immediately clear who was responsible for the attack on the post at Chinagodrar, about 130 miles (209 km) north of the capital Niamey, in which 63 assailants were also killed.
But the attack coincides with a campaign by Islamist groups connected to al Qaeda and the Islamic State group to force the Nigerien army back from its western frontier with Mali where government control of the rural centre and north has all but evaporated because of the rise of jihadists.
Despite efforts by international forces to stop them, attacks have risen fourfold this year in Niger, killing nearly 400 people, according to data from the Armed Conflict Location & Event Data Project, a non-profit research organization.
This included a raid last month that killed 71 soldiers at another military outpost about 150 km to the west of Chinagodrar, the biggest on the Niger military in living memory.
Security has deteriorated this year across the Sahel, a semi-arid strip of land beneath the Sahara, amid jihadist attacks and deadly ethnic reprisals between rival farming and herding communities.
The region has been in crisis since 2012 when ethnic Tuareg rebels and loosely-aligned jihadists seized the northern two-thirds of Mali, forcing France to intervene the following year to beat them back. The jihadists have since regrouped and expanded their range of influence.