The air strike was carried out Friday in self-defence after fighters were “observed manoeuvring on a combined patrol,” while the US also responded with “indirect fire”, a military spokesman said on Saturday.
No US or Somali forces were killed or wounded in the attack, AFRICOM spokesman Nate Herring told The Associated Press.
The confrontation occurred about 50km northwest of the port city of Kismayo, the US Africa Command statement added.
Two other al-Shabab fighters were killed by Somali forces with small arms fire during the engagement, it said.
The US has carried out more than 20 air raids this year against al-Shabab.
US military involvement in Somalia has grown since President Donald Trump early in his term approved expanded operations against al-Shabab. Dozens of drone attacks followed.
Late last year, the military also carried out its first air raid against a small presence of fighters linked to Islamic State of Iraq and the Levant (ISIL) in northern Somalia.
Since the expanded operations, two US military personnel have been killed in Somalia.
A soldier was killed in May 2017 during an operation about 64km west of Mogadishu.
And in June, one American commando was killed and four US troops wounded in an “enemy attack” as Somali and Kenyan forces came under mortar and small-arms fire in Jubaland.
The US currently has about 500 military personnel in the Horn of Africa nation.
Al-Shabab was pushed out of the Somali capital, Mogadishu, in recent years but continues to control rural areas in the south and central regions.
The al-Qaeda-linked group is fighting to overthrow the UN-backed government and has been carrying our bomb and gun attacks for more than a decade.
Its fighters continue to attack the bases of a multinational African Union force that remains largely responsible for security, as Somalia’s fragile central government tries to recover from decades of chaos.
In the next few years, Somali forces are expected to take over responsibility for the country’s security as the AU force withdraws.
Concerns about their readiness remain high, and the UN Security Council recently voted to delay the handover’s target date to December 2021.