The blast occurred in the central Hiran region as Al-Shabaab members attempted to plant explosives in a vehicle intended for use in a planned attack on local government offices, a Somali army officer told a radio station affiliated with the African Union Mission in Somalia (AMISOM).
It was the second explosion in three days to occur in an area controlled by Al-Shabaab.
A similar blast took place last Friday in another Somali city, killing several al-Shabaab members as they attempted to transport ammunition and military equipment, according to a military official.
The Al-Shabaab movement has yet to comment on Monday's blast.
Observers say Al-Shabaab, which has recently seen the balance of power tilt towards AMISOM-backed government forces, wants to retain momentum by striking military and government targets across the country.