Activists say an explosion struck a vehicle packed with passengers traveling in southern Syria overnight, killing at least 21 people, including four children.

The Britain-based Syrian Observatory for Human Rights said Wednesday that the blast around midnight hit the vehicle as it was driving near Tel al-Juma in Daraa province.

The Observatory says six women were also among the dead.

It was not immediately clear what caused the explosion, but activists in the area accuse the military of planting a bomb in the ground along the road.

The Observatory says there is an army outpost in the area that is besieged by rebels.

Daraa was the birthplace of the Syrian uprising against President Bashar Assad in March 2011.