Officially, the boat was listed as taking about 180 passengers, but Lambert Mende, the communications minister, said that there was likely to be more onboard.
"There are too many people who are in the habit of travelling clandestinely," he said. "Since there were certainly clandestine passengers, other people will have fallen into the water."
Mende said that naval forces and local Red Cross officials had gone to assist and collect the bodies
Last year, at least 73 people were killed after a boat sank on Lake Mai Ndombe in Bandundu province.
Despite its vast mineral wealth, roads are almost non-existent outside the country's main towns.
Travel is often limited to aircraft and the boats that ply its huge network of rivers.
Accidents are frequent due to overloading, lack of maintenance, and lax enforcement of safety standards.