At least six Somali children have died after picking up explosives in their school playground, a local official has told the BBC."I heard a very loud explosion, and minutes later I saw mothers crying,” witness Abdifatah Mohamed told AFP.The children were aged from five to 10. Another four were injured in the explosion in the town of Balad, which was recently seized from Islamist militants. It is 30km (20 miles) north of the capital, Mogadishu.The explosion reportedly happened shortly after school opened on Monday.Somalia has been convulsed by two decades of conflict.Although pro-government forces have gained much territory recently, al-Shabab, which has joined al-Qaeda, still controls many southern and central areas of the country.The deaths come as al-Shabab was forced out of the key port of Merca - one of the largest in the country.Under a UN-backed peace process, a new parliament is due to choose a president in the coming weeks.