The African Union peacekeeping mission in Somalia (AMISOM) has apologised for accidentally killing four civilians after soldiers mistook them for al-Shabab fighters when they failed to stop at a roadblock.

The apology came on Sunday, a day after the incident happened in the area of Buulo Mareer in the Lower Shebelle region, 80km south-west of the capital, Mogadishu.

The dead include an 80-year-old woman and her nine-year-old granddaughter who were sick and were travelling to Mogadishu for treatment.

The AU Mission said scared soldiers had opened fire when the car failed to stop at a roadblock.

Abdiwahid Ibrahim Maalim, the son of the elderly woman killed, said she and the granddaughter were in the car with two of his friends, one of them the driver, when the troops killed them.

Residents of Lower Shabelle have protested, denouncing the killings.

AMISOM spokesman Col Joe Kibet told BBC Somali that the driver had defied an order to stop.

Amisom has more than 22,000 soldiers and police, the majority from six African countries, deployed in Somalia to protect the government there.

•Based on reports by Pan African Vision and BBC. Photo shows AMISOM troops on duty.