"No one knows what this attack may mean in the future, but the fighters have always tried their best to target government troops and those of the AU."

Al Jazeera's correspondent in Mogadishu said that the shelling was initiated by the Mujahidin Youth Movement, an opposition group that wanted to prevent the arrival of military supplies to AU forces.

The AU mission in Somalia, which is the sole foreign force after the pullout of Ethiopian soldiers earlier this year after a two-year intervention, has come under repeated attacks by Somali fighters.