"(Warship) KRI Hiu saved 13 survivors just south of Bawean island today (Wednesday)," Central Java navy commander Colonel Yan Simamora told AFP.

Bawean is 350 kilometres northeast of the Javanese port of Semarang, the original destination of the ill-fated ferry.

Navy ships, fishermen and search and rescue teams have found more than 200 survivors since the ferry sank late Friday with some 600 people on board.

"So far there are 205 survivors and seven people found dead," Semarang port official Bagasto told AFP.

Three unidentified victims were buried Wednesday.

The large wooden coffins were lowered into the ground in a simple ceremony in Rembang, east of Semarang, television pictures showed.

Survivors have been spotted on liferafts but bad weather and huge waves have been hampering efforts to pluck them to safety after days on the open sea.

Food and drinks have been dropped to them, attached to tyres.

"The weather is still relatively bad, with strong winds and rain," Simamora said, stressing that the search would continue despite the adverse conditions.

"We have seven navy ships, helicopters and Nomads (patrol aircraft) currently searching the area as far east as north of Madura island," just off Surabaya in East Java province, he said.

Meteorologists have warned that the bad weather and rough seas will continue for the next few days, with waves of two to four metres high and strong winds.

Preliminary investigations show bad weather was to blame for the accident, transport safety officials have said, as they continue to collect data.

The "Senopati Nusantara" (Archipelago Commander) was en route from Kumai on Borneo island to Semarang when it sank.

The vessel was carrying 542 passengers and 57 crew when it sank. It was licensed to carry 850.