The multinational force "came under mortar and small-arms fire at approximately 2:45 pm Mogadishu time (1145 GMT), killing one USA service member and injuring four U.S. service members and one partner force member", the United States military's Africa Command said in a statement. "Names are being withheld pending next of kin notification".
"My thoughts and prayers are with the families of our serviceman who was killed and his fellow servicemen who were wounded in Somolia (sic)", he tweeted.
The coalition forces were conducting a "multi-day operation" to clear al-Shabaab - an Islamist militant group - from villages and establish a "permanent combat outpost" around 217 miles southwest of Mogadishu. The US estimates that al-Shabaab commands 4,000 to 6,000 active fighters, making it one of the terror group's largest affiliates.
"The population in the region had historically supported the government, and the Somali forces had prepared for this mission by coordinating heavily with and securing the support of local authorities ahead of time", U.S. Africa Command said.
At least one USA special forces solider was killed and four U.S. service members were wounded after an enemy attack in Somalia.
"One of the wounded US service members received sufficient medical care in the field", the statement says.
Although one of the wounded Americans did not receive additional care after being treated in the field, the other three and the wounded local soldier were medically evacuated for follow-up care.More news: GOP leader McConnell cancels most of Senate's August recess
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U.S. forces were offering "advice, assistance and aerial surveillance during the mission", U.S. Africa Command said, noting that the mission was created to increase the Somali government's ability to provide services to innocent civilians under al-Shabab rule.
USA military operations in Africa have come under greater scrutiny since an October 4 ambush by Islamic State militants in the West African country of Niger left four US soldiers dead.
Traditionally, US presidents have been wary of intervening in Somalia since 18 special forces soldiers died fighting militias in Mogadishu in 1993, a battle dramatised in the film Black Hawk Down.
Trump approved expanded military operations against al-Shabab in early 2017, leading to an increase in USA military personnel to more than 500 and the launch of dozens of drone strikes.
"We have increased the firepower; we've increased the ISR [intelligence, surveillance and reconnaissance] capacity; we've increased various response times", Marine Gen. Thomas Waldhauser, head of AFRICOM, told reporters on May 10.
A Pentagon investigation into the Niger attack, parts of which were made public last month, found multiple failures but none that directly caused the ambush by Islamic State group-linked fighters.