"So far we know of seven people who died, mostly civilians, but the death toll may rise," said Major Ali Umar.
"Over a dozen others were injured, mostly Somali military personnel."
A medic at Galkayo hospital said it had received over 30 wounded as a result of the explosion.
The terrorist group al-Shabab, which is trying to topple the central government, could not be reached for comment.
The group is fighting to seize power and establish a state based on its own beliefs. It controls small sections of the Mudug region, but not Galkayo.