The incident happened in Kismayo, located 528 kilometers southwest of the Mogadishu on Wednesday.
The attack also destroyed many houses in the area, said local sources.
The US military uses remote-controlled drones in Somalia for reconnaissance operations and targeted killings.
Washington has been carrying out assassination attacks using the unmanned aircraft in other countries including Afghanistan, Libya, Pakistan, and Yemen.
The United States claims the CIA-run strikes are aimed at militants. But witness reports and figures offered by local authorities indicate the attacks have led to massive civilian deaths.
The UN has condemned the US assassination drone strikes, saying they pose a challenge to international law.
The weak Western-backed transitional government in Mogadishu has been battling al-Shabab fighters for the past five years, and is propped up by a strong African Union force from Uganda, Burundi, and Djibouti.
Strategically located in the Horn of Africa, Somalia remains among the ones generating the highest number of refugees and internally-displaced persons in the world.