The U.S. military on Friday published the names of the three Army Special Forces soldiers from 3rd Special Forces Group (Airborne) who were killed when their reconnaissance patrol with Nigerien forces came under fire.
According to the sources, one soldier form Niger was also killed in the attack.
The Associated Press reports four Nigerien soldiers were also killed and others from the USA and Niger were wounded in the attack believed to have been carried out by Islamic extremists.
"A joint patrol of the Defense and Security Forces and American partners operating in the border area of Mali fell into an ambush set by terrorist elements aboard a dozen vehicles and about twenty motorcycles", the statement said. "This is about black market resources which terrorists use to gain revenue that helps them recruit, expand and export attacks", the official said.
Military officials aren't sure how the missing soldier became separated from the rest of the troops but they do not believe he was captured, CNN says.
The American service member, who has been identified by United States military personnel, went missing after a group of USA soldiers near the Niger-Mali border were attacked by up to 50 fighters that are thought to be likely affiliated with ISIS, a U.S. official said.
Asked whether the USA forces had adequate protection, including readily available medical evacuation, McKenzie said rapid evacuation is a primary concern when developing any US mission.
The suspected militants who launched the raids belong to the al-Qaeda in Islamic Maghreb - a group affiliated to the al-Qaeda that is active in the North Africa region.
The U.S. Africa Command based in Stuttgart, Germany, said the U.S. forces are in Niger to provide training and security assistance to the Nigerien armed forces in their efforts against violent extremists.
"Our thoughts and prayers are on the families of the deceased", wrote the Army's Africa Command in a statement. "I don't have the exact numbers, but yeah, because Niger is an important partner of ours".
White House Press Secretary Sarah Huckabee Sanders told reporters Wednesday night that Gen. John Kelly had briefed President Donald Trump on the incident.
Islamist militants, including al-Qaeda fighters, operate in the region.
In Niger, Washington has deployed around 800 soldiers, runs a drone base in the capital Niamey and is building a second in Agadez at a cost of around $100 million.