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.
The attack happened 200km (124 miles) north of Niamey, the capital of Niger, the US Africa Command statement said.
The two injured US soldiers had been evacuated to the Landstuhul Regional Medical Centre in Germany and were in a stable condition, it added.
According to the New York Times, those killed were special forces soldiers and were the first Americans to die as a result of hostile fire since the US Africa Command deployed in Niger.
Earlier, an official from Niger's Tillaberi region told Reuters that five Nigerien soldiers were among the dead.
It is not clear who fired on the soldiers.
Militants belonging to al-Qaeda in the Islamic Maghreb (AQIM) - an affiliate of al-Qaeda - are active in the region and are known to have conducted cross-border raids.