The Indonesian military continued with a search and rescue operation on Lake Toba on Wednesday, after a ferry sank there early this week.

Distraught relatives slammed Indonesia's government for not enforcing basic safety measures on passenger boats and pleaded for a bigger search effort for at least 178 people still missing.

The wooden vessel, overcrowded with passengers as well as dozens of motorbikes, didn't have a manifest and disaster officials have several times raised the number of people it was carrying.

Only 18 survivors have been found - in bad weather within hours of the sinking on Monday evening.

It's possible many of the victims were still inside the sunken ferry, said North Sumatra province police chief Paulus Waterpau.

Divers were searching at depths of 25 metres (82 feet) and will deploy an underwater drone to 200 metres (656 feet) below the surface, Waterpau said.

The 1,145-square-kilometre (440-square-mile) Lake Toba, formed out of an ancient super volcano, is a popular sightseeing destination on the island of Sumatra.

It's more than 400 metres (1,312 feet) deep.

The search and rescue effort involving 350 personnel and at least half a dozen boats has turned up items of clothing, bags and traces of oil from the ferry but just several victims.

A spokesman for the search and rescue agency in nearby Medan said three deaths were confirmed after two more bodies were found on Wednesday morning.

The disaster has cast a tragic pall over the holidays marking the end of Ramadan when tens of millions of Indonesians return to their hometowns.

Ferry tragedies are common in Indonesia, an archipelago of more than 17,000 islands, with weak enforcement of safety regulations often to blame.