Three US soldiers have been killed and two others wounded in an ambush in Niger near the border with Mali, the US Africa Command has said.
Another soldier from a “partner nation” had also died in the attack, it said – without specifying their nationality.
The US soldiers had been providing advice and assistance in Niger’s counter-terror operations and come under “hostile fire”, it said.
Islamist militants, including al-Qaeda fighters, operate in the region.
They are most active in neighbouring Mali, where French troops intervened in 2013 to prevent them from advancing on the capital.
The Boko Haram group, based in Nigeria to the south, has also staged several attacks in Niger.
US President Donald Trump has been briefed by his chief-of-staff, John Kelly, about the attack, White House spokeswoman Sarah Sanders said.