The troops had been on a mission to clear al-Shabaab from contested areas as well as villages the militants controlled, "and establish a permanent combat outpost" to expand the reach of the Somali state, the US military's Africa Command said in a statement.
The military said the USA service members were part of a 800-member force of Somalia and Kenya troops that came under came under mortar and small-arms fire in Jubaland, about 200 miles southwest of the capital, Mogadishu.
Friday's joint operation, part of a multi-day mission including about 800 Somali and Kenyan troops, aimed to clear al-Shabab from contested areas.
We "determined the allegations of civilian casualties to be not credible", officials said in a statement.
A suicide auto bomb explosion at a military base in Somalia injured seven soldiers late Saturday, a military official said, and Islamist group al Shabaab claimed responsibility for the attack.More news: I tried out Apple’s new HomePod features. Here’s what I learned
Names of the soldiers have not been released while the USA notifies next of kin.
President Donald Trump paid tribute on Twitter on Friday night, offering "thoughts and prayers" to the families of the soldier who was killed and those who were wounded. One coalition service member was wounded. They are truly all HEROES. The U.S. provided advice, assistance and aerial surveillance during the mission, the statement said.
More than 500 American forces are partnering with African Union Mission to Somalia (AMISOM) and Somali national security forces in counterterrorism operations, and have conducted frequent raids and drone strikes on Al-Shabaab training camps throughout Somalia.
It was the first public announcement of a USA military combat death on the continent since four US service members were killed in a militant ambush in the west African nation of Niger in October. The overarching goal in Somalia for the Defense Department is to help the Somali government in providing a safe and secure environment for the Somali population, they added. The group was pushed out of the capital, Mogadishu, in 2011 but still controls some regions of the country.