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More people are missing from the Monday accident in Lake Mai-Ndombe, the Mayor of Inongo Cosmos Mboo Wemba is quoted as saying.
The cause of the accident is not yet known.
The vessel was carrying around 30 people who were returning from a funeral 35 kilometres (20 miles) away, journalist Firoger Balimba is quoted by AFP as saying.
In May 2019, at least 45 people died in the lake after a boat that was carrying more than 400 people sank.
The majority of those on board were teachers going to collect their salaries.
Boat accidents are frequent in DR Congo because of overloading and poor maintenance.