The airstrike was carried out about 40 miles southwest of Somalia’s capital, the US statement said, adding that no civilians were killed.
It was not immediately clear how many al Shabab fighters may have been killed.
A senior Somali intelligence official said the airstrike largely destroyed a minibus travelling near the rebel-held village of Mubarak in Lower Shabelle region.
The al Qaida-linked al Shabab has been blamed for the massive lorry bombing in Mogadishu in October that killed 512 people, wounded more than 300 others and left more than 60 missing.
Only a few attacks since the ones on September 11 2001 have killed as many people, according to the Global Terrorism Database at the University of Maryland.
Al Shabab, the deadliest Islamic extremist group in Africa, often targets high-profile areas of Mogadishu including hotels, military checkpoints and the presidential palace.
The US military has carried out 32 airstrikes this year against the Somalia-based al Shabab and a small but growing presence of fighters linked to the Islamic State group.
The Trump administration early this year approved expanded military operations against extremists in the Horn of Africa nation, with counter-terrorism at the top of its foreign policy agenda for Africa.
More than 500 US military personnel are now in Somalia, the US Africa Command said last month.