The boat was taking people from Monkoto to the city of Mbandaka when it capsized, the vice governor of Tshuapa province said.
A Congolese official has said that the cause of the accident remains unknown.
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There were about a hundred people in the boat — about half of them have been rescued.
Such emergency situations often occur on the rivers of the Congo because of the congestion and the age of boats and vessels. Water arteries are often the only means of movement for local residents.