The Pentagon has received permission from the government of Niger to begin flying armed drones from the capital, Niamey, a U.S. official said Friday.

The arrangement, which has not been publicly announced, reflects an expanding U.S. military campaign against extremists in Africa and is based on a recently signed U.S-Nigerien memorandum of understanding. It was first reported by the New York Times.

The U.S. official said armed drone flights could begin as early as next week or at least by the end of December. The memorandum of understanding limits the drones to defensive missions, the official said. The official was not authorized to discuss the matter publicly and so spoke on condition of anonymity.

A Pentagon spokeswoman, Army Maj. Audricia M. Harris, said she could not comment on drone arrangements.

"The government of Niger and the U.S. stand firm in working together to prevent terrorist organizations from using the region as a safe haven," she said. "For operational security reasons, I will not comment on specific military authorities or permissions."

The U.S. military has been targeting a number of extremist groups in Africa, including Boko Haram, al-Shabab, the Islamic State group and an IS splinter group known as Islamic State of the Sahel. The U.S. Africa Command has accelerated airstrikes against al-Shabab fighters in Somalia in recent weeks.

The drones at Niamey eventually are to be moved to a desert base under construction near Agadez. That base is expected to become operational by mid- to late-2018.

The U.S. has approximately 800 troops in Niger. That presence drew greater U.S. public attention after an Oct. 4 attack by extremists killed four U.S. and four Nigerien soldiers.

The U.S. is in the midst of an investigation to determine how the attack happened, why one of the U.S. soldiers got separated from his partners and what mission they were performing at the time. Families of the four Americans who died have been told the probe is unlikely to be finished before January.