The bomber struck shortly after noon in front of a police checkpoint in Janale, Lower Shabelle region.
Witnesses said a grenade attack killed three people, including two NISA soldiers in Janale and a 12-year-old boy, at the checkpoint.
The sources added that the troops conducted an operation.
Reports say that the man, who was shot dead in Janale area, was arrested several times on suspicion of collaborating with Al-Shabaab, and when he was killed he refused to stand by the troops.
Janale has been the scene of sporadic bombings and direct fighting, and operations against al-Shabaab have been ongoing in recent days.