A Tanzanian ferry carrying at least 250 people has sunk near the island of Zanzibar.
The navy said the vessel, the MV Skagit, got into difficulty because of strong winds.
Last September, nearly 200 people died when an overcrowded boat with 800 people aboard sank off Zanzibar.
The BBC's Aboubakar Famau in Dar es Salaam said the MV Skagit left the mainland at 12:00 local time (09:00 GMT) bound for the main island of the semi-autonomous archipelago.
The journey usually takes about two hours.
Thirty-one children are believed to have been on board, our reporter says.
"The ferry is bottom-up, and rescue operations are ongoing. So far we have not confirmed any deaths," Said Khamis, a safety officer at the Zanzibar Port Corporation, told Reuters news agency.