Three U.S. soldiers were killed and two wounded in an ambush on a joint patrol in southwest Niger on Wednesday.
The five Green Berets were attacked while on a routine patrol in an area known to have a presence of insurgents, including from al Qaeda in Islamic Maghreb (AQIM) and Islamic State, according to Nigerien and U.S. officials.
Five soldiers from Niger were also killed during the attack.
It was unclear who fired on the U.S. and U.S.-backed forces.
Those forces were not patrolling the area with any specific objective, such as a high-value target or rescuing a hostage, a spokesman for U.S. Africa Command said.
A Niger diplomatic source said the attackers had come from Mali and had killed several soldiers, without saying whether any of the U.S. troops stationed in the West African country were among the victims.
African security forces backed by Western troops are stepping up efforts to counter jihadist groups forming part of a growing regional insurgency in the poor, sparsely populated deserts of the Sahel.
A relatively new militant group called Islamic State in the Greater Sahara has claimed some of the attacks.
The U.S. Africa Command has hundreds of soldiers deployed across the region, including at an air facility in Agadez, and offers training and support to Niger’s army in aspects such as intelligence gathering and surveillance.