Rescuers plucked more than 230 survivors from the sea off Papua New Guinea's east coast after a ferry sank Thursday with as many as 350 people on board, officials said.

An airplane from Australia, three helicopters and eight ships were scouring the search area after the MV Rabaul Queen went down around dawn while traveling from Kimbe on the island of New Britain to the coastal city of Lae on the main island, Australian Maritime Safety Authority said in a statement.

The ferry sank 50 miles (80 kilometers) east of Lae, the South Pacific country's second-largest city, and 10 miles (16 kilometers) from shore, the statement said.

Papua New Guinea Prime Minister Peter O'Neill said the cause of the accident was unknown.

He said more than 300 people were aboard the ship, although the precise number had yet to be confirmed.

Australian Broadcasting Corp. quoted police in Kimbe, where the ferry sailed from, as saying most of the passengers were students and trainee teachers.

The Australian authority's dedicated search and rescue airplane based in the northern city of Cairns had reached the scene by afternoon and another two Australian airplanes were on their way.

The authority's spokeswoman Carly Lusk said the crew of the first airplane threw several life rafts to survivors in the water.

She did not know whether the search would continue into the night.

In the hours just after the sinking, Australian Prime Minister Julia Gillard called the incident a ``major tragedy.''

``Given the likely very high loss of life here, I think when this news comes to the attention of Australians around the country they will be thinking about the people of PNG as they respond to this tragedy,'' she added.

The Australian maritime agency initially detected the ferry's distress beacon and alerted the PNG Maritime Rescue Coordination Center, which is coordinating the rescue effort. Lusk said 238 survivors had been recovered by late Thursday.

She said 350 people were believed to be on board, but Papua New Guinea's National Maritime Safety Authority rescue coordinator, Captain Nurur Rahman, said the true figure was likely lower.

``I cannot confirm or deny the 350 missing number. It is hearsay,'' Rahman said. ``I have not seen the manifest as yet, but it is likely around 300.''

Rahman said there had been no reports of bodies being found.

Ship operator Star Ships could not be immediately contacted for comment.