"Most of the dead are civilians. They are women and children," Hassan-Kafi Mohamed Ibrahim, deputy police commissioner of Hirshabelle State, told Reuters.
"Only one child survived from a family of nine members. Other families also lost half of their members. The two suicide car bombs burnt many civilian homes to ashes."
Mahas District Commissioner Mumin Mohamed Halane told state radio that one bomb targeted his house and the other hit the home of a federal lawmaker.
Al Shabab’s media office claimed responsibility in a statement, saying it had targeted "apostate militias and soldiers" and put the number of dead at 87.
Al Shabab often gives higher casualty figures than local officials and residents.
Al Shabab has been waging an insurgency against Somalia's government since 2007. It was pushed out of Hiraan, the region where Mahas is located, last year by government forces and allied clan militias known as macawisley.
President Hassan Sheikh Mohamud's government says the operations have killed hundreds of al Shabab fighters and recaptured dozens of settlements, although many battlefield claims cannot be independently verified.
Despite the offensive, al Shab\ab has carried out frequent attacks in recent months, including several in the capital Mogadishu against government installations and hotels.
Al Shabab's activities have also restricted deliveries of international aid, compounding the impact of the Horn of Africa's worst drought in four decades.