At least five Somali children were killed and more than 10 others wounded in a large blast in a school on Monday, after they played with explosives left over from fighting in the area, officials said.

"There was a heavy explosion, we believe caused by an explosive device that they had been playing with," said Abdi Jinow Alasow, governor for the Middle Shabelle region.

"At least five children have died and more than 10 others were injured," he added. "The death toll could rise as most of the victims have suffered serious injuries."

The explosion in the town of Balad, some 30 kilometres (20 miles) north of the capital Mogadishu, took place shortly after the school opened on Monday morning.

The town was recently wrested from the control of Al-Qaeda-linked Shebab insurgents by African Union troops alongside government forces.

"I heard a very loud explosion, and minutes later I saw mothers crying," said Abdifatah Mohamed, a witness.

"There were nearly 20 children who were attending the class when the explosion occurred, most of them were either killed or seriously injured."