Early on Friday, a fire broke out at the third-floor apartment of a six-storey building at the port city's Dewanbazar.
Locals said they heard a loud explosion before the fire started.
Fire fighters put out the flames and sent three persons to Chittagong Medical College Hospital (CMCH).
CMCH burns unit Assistant Professor S Khaled said they died between 12.30am and 1am.
Ifti's brother 'Arif', 26, who was also being treated at the hospital, has been transferred to the Dhaka Medical College Hospital, said Khaled.
A gas leak from an unlocked stove in the kitchen caused the explosion and fire, according to police's bomb disposal unit.
Almost all the window glasses and doors of the house were ripped apart by the blast. The walls of the apartment also cracked.