At least two people have been killed in an attack carried out by a US assassination drone in southern Somalia, an al-Shabab militant and a senior US military official have said.

A member of the militant group said the unmanned aircraft fired a missile at a car in Somalia's Middle Juba region on Monday, the Associated Press reported.

The US official, who spoke on condition of anonymity, claimed the vehicle was carrying senior members of al-Shabab.

The US military uses remote-controlled drones in Somalia for reconnaissance operations and targeted killings.

Washington uses assassination drones in several countries -- including Afghanistan, Pakistan, and Yemen -- to target what it calls terrorists. According to witnesses, however, the attacks have mostly led to civilian casualties.

On October 22, a joint report by Amnesty International and Human Rights Watch said US officials could be found guilty of war crimes for the secret CIA drone attacks which have killed hundreds of people.

The aerial attacks, initiated by former US president, George W. Bush, have escalated under President Barack Obama. President Obama recently defended the use of controversial drones as “self-defense.”

The United Nations says the US-operated drone strikes pose a growing challenge to the international rule of law.