Al-Shabaab militants Flee key town as Somali army Advances
The blast hit the 32-seater bus near the Eastleigh suburb, which is home thousands of ethnic Somalis.
No group has said it was responsible for the attack.
But the Eastleigh area has suffered several attacks in the past blamed by officials on Somalia
Sources say that the United States and Turkish forces based in the capital Mogadishu have bombed Al-Shabaab's defensive bases from the air, which forced the militants to flee.