A United States military strike has hit a car carrying senior members of the al-Shabab armed group in Somalia, killing at least two people including the group's top explosives expert, a rebel and a US intelligence official said.

The senior US official told the Associated Press news agency that there was a US strike against a target in Somalia on Monday but would give no further details. The official spoke on condition of anonymity because there was no authorisation to discuss the strike publicly.

An al-Shabab member who gave his name as Abu Mohamed said one of those killed was al-Shabab's top explosives expert, known as Anta.

He said a drone fired at the car in Somalia's Middle Juba region. It was not clear how Mohamed would know it was a drone strike, except that he said no attack helicopters were seen.