US forces provided advice, assistance and aerial surveillance during the mission.
One member of the "partner forces" was wounded along with four other USA troops.
One American special operations soldier was killed and four were wounded in a firefight with al-Shabaab terrorists in southwestern Somalia on Friday.
Though U.S. -backed operations and airstrikes have dented al-Shabab, the militants have continued to carry out deadly attacks in Somalia, including a bombing in October of previous year that left some 300 people dead in the capital of Mogadishu.
The fourth wounded USA service member was treated in country, the Pentagon said.
The coalition force was conducting a "multi-day operation" to clear al-Shabaab, an Islamist militant group, from nearby villages.
Around 500 USA forces are stationed in Somalia.
The al Qaida-linked al-Shabaab extremist group, which is based in Somalia and controls parts of the country's rural south and central regions, claimed responsibility.
The most recent United States casualty in Somalia occurred a year ago, when a member of the Navy SEALs was killed in a nighttime attack in Somalia.
Associated Press video on Friday showed Somali forces firing weapons from atop pickup trucks in a muddy rural area, and an unidentified USA soldier speaking with local residents. A U.S. Africa Command statement said the four were in the care of the U.S. Embassy medical team in neighboring Kenya.
Al-Shabab, which is fighting to impose Shariah law across Somalia, was pushed out of the capital, Mogadishu, and other major urban cities more than two years ago. Though the Pentagon initially described Miliken as operating behind Somali troops, US officials later acknowledged that USA special operations soldiers had been fighting together with the Somali forces. Names of the soldiers were not immediately released, while next of kin were notified.
The US troops were on a training mission with local Somali and Kenyan forces when the attack occurred.
The October attack in Niger raised questions in Washington about the us military presence across Africa as the Trump administration focuses counterterror efforts on a range of groups linked to Al Qaeda and the Islamic State group. A Pentagon investigation of the incident produced a highly critical report.