A group of about 10 to 12 American service members were accompanying Nigerien soldiers on a regular patrol late Wednesday afternoon when they came under attack according to the U.S. official.
Three American service members were killed in the ambush and two others were wounded, said the official. The wounded Americans were taken to Niamey, Niger's capital, for medical treatment.
U.S. Africa Command acknowledged that U.S. forces had come under attack on Wednesday, but did not confirm any U.S. casualties.
"We can confirm reports that a joint U.S. and Nigerien patrol came under hostile fire in southwest Niger," said Lt. Commander Anthony Falvo, a spokesman for U.S. Africa Command. "We are working to confirm details on the incident and will have more information as soon as we can confirm facts on the ground."
There are about 400 U.S. military personnel in Niger most of whom work at two drone bases, in Niamey and Agardez in northern Niger, providing reconnaissance information to the Nigerien military to counter violent extremist groups.
The U.S. personnel who came under attack were part of the effort to help the Nigerien military improve their counter-terrorism capabilities.