The incident occurred on Friday 50 kilometers (30 miles) northwest of Kismayo in the south of the Horn of Africa country where Somali commandos work side-by-side with the American forces that train them.

Air assaults and missile strikes have increased in recent months.

"The US airstrike was conducted against militants after US and partner forces came under attack," US Africa Command said in a statement.

US forces also partner with the United Nations-backed African Union Mission in Somalia, which aims to counter the threat from Al-Shabaab jihadists.

The air strike was conducted "in self-defense," killing 18 militants, Africa Command said.

Two other militants "were killed by Somali forces with small arms fire during the engagement," it added.

In June an American commando was killed during an attack in southern Somalia that also wounded four US military members and a Somali soldier, officials said at the time.

More than 500 American personnel are partnered with African Union Mission in Somalia (AMISOM) and Somali national security forces.

Al-Shabaab, an Al-Qaeda affiliate, is fighting to overthrow the internationally-backed Somali government in Mogadishu.