At least four people were killed when a ferry carrying 240 people sank in bad weather off the Indonesian island of Sumatra Sunday, officials said.

Search teams using a number of naval and other vessels continued to scan the waters for those still missing.

Riau police chief Puji Hartanto told MetroTv that survivors were seen floating in the water in the aftermath of the accident.

The Dumai Express 10 was sailing from Batam near Singapore to the island of Dumai in Riau, Sumatra when it was battered by heavy waves and sank.

Director General of Sea Transportation, Sunaryo, said the death toll increased to four by late Sunday afternoon.

'The data is still confusing, we are not sure (of the number of victims),' Sunaryo told a press conference, adding that reports from the scene estimated that 202 passengers survived.

A police officer who identified himself only as Edwin, meanwhile, told TVOne television that by sunset, at least 19 people were killed in the accident.

The ferry had been carrying 240 people, including 15 children and 12 crew, when it sank, according to Riau marine police officer Yassin Kosasih.

Bad weather and high waves were hampering the search and rescue operation, the state-run Antara news agency reported.

Sunaryo said the cause of the accident was still not known. An investigation was underway to determine whether negligence on the part of the master or the port had played a role.

Another ferry, the Dumai Express 15, with 278 people on board, ran aground after it was hit by large waves on Sunday, police said.

All passengers and crew survived. The ferry had been sailing between Batam and Moro island.

Hundreds have in recent years been killed in ferry accidents in Indonesia. The vessels are often overcrowded, with safety regulations poorly enforced.

The vast country spans more than 17,000 islands and boats are a popular and relatively cheap form of transport.

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