About 100 people are feared trapped inside a ferry that sank in rough weather off Papua New Guinea, a rescue official said Friday, a day after the crowded boat went down with about 350 people on board.

Battling strong winds and high seas, merchant ships backed by Australian rescue aircraft returned Friday to search for survivors where the MV Rabaul Queen sank off the South Pacific's nation's northeast coast.

Rescuers plucked scores of people from the sea Thursday after the ferry was hit by three large waves and quickly sank, Rony Naigu, a rescue official from Papua New Guinea's maritime safety authority, said.

While a definitive passenger list was not available, rescuers believed 352 people were on board the ferry when it sank and about 100 people were thought to have been trapped inside, Naigu told Reuters by telephone.

We think about 100 didn't get out in time....Those are the numbers we are working to now, he said.

Naigu, who spent Thursday at the scene, said survivors told how the ferry rolled and sank in deep water after it was hit by the waves in stormy conditions.

Scores of passengers survived in liferafts and by clinging to debris before being picked up by at least six merchant vessels that diverted to the scene by Australian maritime safety authorities.

We found a number of (opened liferaft) capsules with no people in them, Naigu said.

The Australian Maritime Safety Authority (AMSA) said on Friday that nearly 250 people had been rescued and transferred to Lae, the provincial capital and the ferry's original destination.

Weather conditions had improved slightly from Thursday, but winds of 30 knots (55 kmh) were still hampering rescue efforts, a spokeswoman said.

Survivors were generally in good condition, said Nurur Rahman, the acting chief of Papua New Guinea's maritime authority.

As you would expect, people who have been in the water for such a long time have a bit of dehydration, and they are very tired, Rahman told Reuters by telephone.

No bodies had been recovered Thursday before nightfall halted the search and rescuers remained hopeful of finding more survivors Friday, he added.

Sea temperatures between 24-26 degrees Celsius meant people could survive for two days or more, he said.

With communications in the South Pacific nation difficult and little news from the ship's owners and operators, frustrations were rising.

Police in West New Britain, a province of Papua New Guinea, said relatives angry at the lack of information about their family members threw stones at the offices of the ferry operators, Rabaul Shipping, Australian Broadcasting Corporation Radio reported.

There were a lot of people crying and then they wanted to know the fate of their loved ones, the people actually who were on board the Rabaul ship, Inspector Samson Siguyaru said.

The managing director of the ship's owner, Rabaul Shipping, said they still had no information about what caused the 47m (155 ft) ferry to go down.

Papua New Guinea Prime Minister Peter O'Neill has promised a full investigation into the tragedy.

The Australian Maritime Safety Authority said three merchant ships and four rescue aircraft were taking part in the search, near where the 47 metre (155 ft) ship sank about 9 nautical miles (16 km) off Papua New Guinea's northeast coast.

PNG, Australia's nearest neighbour, is largely undeveloped, with poor infrastructure and limited facilities despite enormous resources wealth.

The majority of its six million people live subsistence lives in villages clinging to jungle-clad mountains or scattered around its many islands. The island nation relies heavily on sea transport.

(Reporting by Lincoln Feast in SYDNEY; Editing by Paul Tait)