Congo's government confirmed the incident, but gave a lower toll of 80 and said it may have been caused by low water levels on the river due to the dry season.
The accident took place mid-week on a stretch of the Congo River in the western Bandundu province, about 80 miles east of Kinshasa, the capital of the vast central African nation.
"The boat was badly overloaded and it didn't make the rough waters," Jolly Limengo, provincial inspector of police for Bandundu, told Reuters by telephone on Thursday.
"People here don't know how to swim. Our provisional toll is 138 dead," Mr Limengo added.
Naval forces and local Red Cross have gone to assist and collect the bodies, said Lambert Mende, the information minister.
Congo is a vast country of jungles and huge rivers in Central Africa. Decades of conflict and neglect have left the nation's infrastructure in tatters, with little more than 300 miles of paved road. Many people prefer to take boats even if they do not know how to swim. The boats are often in poor repair and filled beyond capacity.
In May, dozens of people died when an overloaded canoe capsized on a river in eastern Congo. And last November, at least 90 people were killed after a logging boat sank on a lake in Congo. The timber-carrying vessel was not supposed to be carrying passengers.
As many as 140 people are feared dead after a boat sank on the , a tributary of the , in the Democratic Republic of the Congo.

The central African country, of equivalent size to western Europe, has very poor road infrastructure, meaning that many people travel on boats, which are often overloaded. The boat involved in the accident was travelling to the capital city, , from the town of . The accident occurred in the province of , approximately 30 km (20 miles) from the provincial capital, where officials reportedly held a crisis meeting to deal with the incident.

The boat is believed to have been overweight, carrying at least 180 passengers as well as goods.  It is the dry season in the Congo, so the river is shallow.  The sinking was reportedly the result of hitting a mud bank, causing the vessel to capsize. 

Lambert Mende, the Congolese information minister, said in a statement that at least 80 people had been confirmed dead while 76 were thought to have survived. However, the local police later announced a provisional death toll of 138, and possibly more.

Safety standards are poor in the Democratic Republic, with overcrowding common on boats, which often do not carry lifejackets and are forced to navigate poorly marked waterways, meaning that fatal accidents are not uncommon in the country.