US forces acting in self defense called in an airstrike on al Shabaab militants in the southern Somalia province of Lower Juba, killing an estimated 18 Islamist extremists, US Africa Command said.
Somali government troops accompanying the US unit killed two other fighters of the al Qaeda-linked group with small arms fire during the clash that took place on Friday, it said.
"The US airstrike was conducted against militants after US and partner forces came under attack," US Africa Command said in a statement on Saturday. "We currently assess no civilians were injured or killed."
The clash took place about 30 miles (50 km) northwest of the Lower Juba port city of Kismayo.
US forces are in Somalia supporting a UN-backed government against al Shabaab, an Islamist extremist group that lost control of most of the country's towns and cities after withdrawing from the capital Mogadishu in 2011.
Meanwhile, a person died and another was injured in two car bombs that exploded in the heart of the Somali capital on Saturday and the Islamist group al Shabaab claimed responsibility for the attacks.
The bombs detonated in two different cars near a main road in the centre of the city. Al Shabaab frequently carries out bombings in Mogadishu and other parts of the Horn of Africa country.
"Two people were injured in the two car bombs. One of them died of the wounds," said Major Mohamed Hussein, a police officer.
Al Shabaab told Reuters they had planted the car bombs and that they were targeting a police official but that he had escaped.
Another police officer, Ahmed Nur, said one of the cars was parked and had nobody in it, while the second was moving and the two people inside it had been injured. One of the injured is the one who later died.
On Friday, at least three people were killed in separate attacks in Mogadishu, including a female university student shot dead in her class by two men armed with pistols.
Al Shabaab is fighting to topple Somalia's Western-backed central government and establish its own rule based on its own strict interpretation of Islamic sharia law.
The group also wants to drive African Union (AU)-mandated AMISOM peacekeepers out of the country.