DAKAR (Reuters) - U.S. forces started deploying armed drones in the west African country of Niger earlier this year to attack Islamist militants, the U.S. military said on Monday.

Niger’s government granted American forces permission last November to arm their drones but neither side had previously confirmed their deployment. Before that, U.S. drones had only been used for surveillance.

The U.S. military presence in Niger has expanded in recent years to an 800-strong force that accompanies Nigerien troops on intelligence gathering and other missions, reflecting U.S. concerns about rising militancy in West Africa’s Sahel region.

An ambush by a local Islamic State affiliate in western Niger last October killed four U.S. soldiers. Jihadist groups based in neighboring Mali have also struck military and civilian targets as far afield as Ivory Coast.

“In coordination with the Government of Niger, U.S. Africa Command has armed intelligence, surveillance, and reconnaissance (ISR) aircraft already in Niger,” a spokesperson for United States Africa Command (AFRICOM) said in an email.

“As a matter of operational security, we do not discuss where strike platforms originate from, nor current or future operations.”

The drones are currently being flown out of a base in the capital Niamey while the military completes construction of a $100 million drone base in the central city of Agadez.

The military views the drones as a cost-efficient way to counter the militants but critics fear that drone strikes will cause civilian casualties and trigger blowback from the local population.