MANILA - Rescuers combed waters off the Philippine capital on Christmas Day for at least 23 people missing after a ferry smashed into a trawler in an accident that called the crew's competence into question.

The coastguard said it planned to spend 10 days searching for survivors and would also look into the fitness and discipline of the crew involved in the crash, which topped off a year of deadly disasters for the struggling nation.

The 79-tonne Catalyn B, a flimsy, wooden vessel taking holidaymakers to their home village on a small island off the mouth of Manila Bay, smashed into 369-tonne Anathalia, a metal-hulled fishing boat, killing at least four people.

Officials are hoping that at least some of the other passengers and crew may have survived and are afloat in tropical waters, in what is usually the calmest period of the year.

"I would say that we should stay there for about 10 days and then find out if we can be successful in (finding) the remaining unaccounted passengers," coastguard chief Admiral Wilfredo Tamayo said after dispatching patrol craft and aircraft to the site.

He told ABS-CBN television that the weather was clear and the seas were calm around the accident site. Rescuers expect these conditions to continue, he said.

Forty-six survivors were fished out of the water on Thursday, along with four bodies.

However some survivors warned that some of the missing could have been trapped inside the vessel, which the coastguard said sank in 67 metres (220 feet) of water within minutes of impact.

"Usually people can survive afloat for two to three days in Philippine waters," Ensign Jhoe Barbasa, a coastguard spokeswoman told AFP.

"But other factors also play a big role. Hunger, injuries or ailments, like hypertension, and the weather, can affect that window," she added. Since there were no reports of engine trouble or bad weather during the pre-dawn collision, Tamayo said that "obviously" someone failed to observe nautical rules on rights of way, leading to the disaster.

"We're also looking at conditions of the watch at the navigating bridge -- whether the crew or the captain are in good physical condition, whether they (were) drunk or tired or maybe they were not awake at that time."

A formal inquiry will start "maybe in three or four days", he said.

The fishing boat has been impounded, Barbasa added.

"We started listing within two minutes of the collision," said 27-year-old survivor Erwin Broncano.

"There was no warning. There was no order to abandon ship. I ran upstairs to rouse the other passengers there. I grabbed a life jacket and did not even have time to tie it properly before I jumped out," he told local radio.

"The wind was up and there was a strong current, but the skies were clear," said Broncano, whose two female cousins and a niece are among the missing.

Ferries form the backbone of mass transport in the archipelago nation of 92 million people. But bad weather, poor maintenance, overcrowding and lax enforcement of regulations have led to frequent disasters.

The world's deadliest peacetime maritime disaster occurred south of Manila in 1987 when a ferry laden with Christmas holidaymakers collided with a small oil tanker, killing more than 4,000 people.

A series of tragedies have struck the Philippines in recent months, including two tropical storms that claimed more than 1,000 lives in September and October and an election-linked massacre of 57 people last month.

More than 47,000 people are spending Christmas in evacuation camps around the erupting volcano Mount Mayon.