Suspected Islamist militant groups 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.

It was not immediately clear who was responsible for the attack on the post at Chinagodrar, about 209 kilometers north of the capital Niamey.

It coincides with a campaign by Islamist groups connected to al-Qaida and ISIL to force the Nigerian army back from its western frontier with Mali, where government control of the rural center and north has all but evaporated because of the rise of jihadists.

Attackers approached the military post on motorcycles and other vehicles and were met with resistance from the army and the air force, the government said in a statement. Sixty-three assailants were also killed during the raid.

"The Defense Ministry offers its condolences to the victims and wishes the wounded a prompt recovery," the statement said, adding that a search operation was underway to find those responsible.

Despite efforts by international forces to stop them, attacks have risen four-fold over the past year in Niger, killing nearly 400 people, according to data from the Armed Conflict Location & Event Data Project, a non-profit research organization.

They include a raid last month that killed 71 soldiers at another military outpost about 150 kilometer to the west of Chinagodrar, believed to be the biggest attack on the Nigerian military.

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.