Militants ambushed an army patrol in Niger on Thursday, killing 15 soldiers and wounding 19, the Defense Ministry said, occurring in an area near the border with Mali where militants have been seeking to expand their reach.

“A search operation is underway to neutralize the terrorists,” the ministry statement said of the attack just outside the town of Tiloa.

Militants and allied criminal gangs have long exploited the areas around the vast, desert border between Mali and Niger to launch attacks, even after a French-led military intervention pushed insurgent groups back from northern Mali in 2013.

Attacks in Niger are much rarer than in Mali, although militants in the past year have been expanding across borders in the fragile Sahel region into Niger and Burkina Faso.

Gunmen riding motor bikes and other vehicles raided a village and killed five soldiers in Niger in November. Assailants kidnapped a U.S. aid worker in central Niger a month earlier.