Khaleda Yasmin, a duty officer of Fire Service and Civil Defense Headquarters, told Xinhua news agency that "the bodies of three boys, a woman and a girl have so far been retrieved".
According to the official, the boat carrying about 18 people capsized following a collision with a sand-carrying vessel in the river in the early hours of Saturday.
Yasmin said most of the passengers were able to swim ashore after the incident.
Some passengers of the vessel are still missing, Yasmin added.
In Bangladesh, ferries are a major means of transport while most of them are often overcrowded.