"Alongside our Somali and global partners, we are committed to preventing al-Shabaab from taking advantage of safe havens from which they can build capacity and attack the people of Somalia", the statement said. There are no reports of Americans killed or wounded.
An intense exchange of gunfire followed but Somali soldiers were able to regain full control of the base located in Lower Juba province, he said.
Earlier on Saturday, at least eight Somali soldiers were killed when al-Shabab fighters overran a military camp on the outskirts of the town of Kismayo.
The operation came in response to an attack by a large group of al-Shabab militants against Somalia National Forces, according to the statement.
On Friday they ambushed an Ethiopian military convoy 80 km (50 miles) north of Baidoa in central Somalia.
The group could not immediately be reached for comment.
In neighbouring Ethiopia, state television cited the defence ministry as saying more than 60 al-Shabab fighters had been killed and that four vehicles loaded with explosives had been "destroyed".
The attacker, one of five al-Shabaab militants who stormed the complex on Tuesday, was identified as 25-year-old Mahir Khalid Riziki, who grew up in the Kenyan port city of Mombasa.
There has been a significant increase in airstrikes in Somalia by the United States since President Donald Trump authorized the U.S. military to carry out precision strikes targeting al-Shabaab in March 2017 in a effort to bolster the Federal Government in Somalia. Some have targeted top al-Shabab leaders or key financial officials; the extremist group funds its attacks with an extensive network of "taxation" and extortion.
The U.S. on Saturday said it is committed to "preventing al-Shabab from taking advantage of safe havens from which they can build capacity and attack the people of Somalia".