The U.S. Africa Command said the Somali government helped coordinate the operation northwest of Kismayo.
"The strike targeted the militants as they mobilized to launch an attack against our base. Fortunately, the military from a friendly nation disabled them with a drone strike and killed 13 of them," a Somali military commander in the area told VOA.
The United States carried out about 30 airstrikes against al-Shabab this year, including a drone targeting a militant training camp last month, which killed 100.
A senior Somali army officer said Wednesday said more than 20 al-Shabab militants, including two mid-level commanders, have surrendered to Somali security forces over the past two weeks.
Militants agree to lay down arms
Abdullahi Isaq Ibrahim, the chief military commander of South West state in Somalia, told VOA the militants agreed to lay down arms and surrender to the Somali National Army in separate groups.
"One of the [commanders] was in charge of finance and alms collection for al-Shabab in the region and the other was in charge of transportation," Ibrahim said.
Ibrahim said former al-Shabab leader Mukhtar Robow persuaded the militants to give up. Robow defected from the group and surrendered to the Somali government in August after negotiations with Somali officials.
"The same guy, Robow, who told them to go against us, is now with people he once recognized as infidels. He told them they should leave the group and it seems they heard it," Irbahim said. "We are expecting more in the coming hours and days, including senior commanders, to defect."
Some of the defectors were welcomed Wednesday in Baidoa, the provincial capital of the Bay region.
Robow was once al-Shabab's No. 1 leader, and the U.S. offered $5 million for information leading to his capture, but he had a falling out with al-Shabab leaders in 2012 and has since kept a low profile in the jungles of Bakool region, protected by members of his clan.
Multiple attempts by al-Shabab to kill or capture him failed.