A naval search operation resumed at first light to find an estimated 100 people missing after a Tanzanian ferry sank near the island of Zanzibar.

The boat had left the city of Dar es Salaam on mainland Tanzania on Wednesday with 290 people on board.

So far 31 bodies have been recovered and more than 150 people rescued.

Zanzibar's police spokesman told the AFP news agency he feared there was now little hope of finding survivors from the MV Skagit ferry.

"Search operations continue but it is now almost impossible survivors will be found," the agency quotes him as saying.

Three days of mourning

Survivors said they fear some of the passengers became trapped in vessel when it capsized in strong winds.

Ali Mohamed Shein, president of the semi-autonomous archipelago of Zanzibar, has declared three days of national mourning.

The ferry left Dar es Salaam at 12:00 local time (09:00 GMT) on Wednesday bound for the main island of Zanzibar.

Hamza Kabelwa, director of the Tanzania Meteorological Agency, told the BBC's Swahili service that a warning had been issued for vessels not to make the crossing because of the rough seas.

Zanzibar's transport minister told reporters that two Europeans were among the dead.