Typhoon Saola pounded Taiwan with fierce winds and torrential rain that left six people dead and forced nearly the entire island to shut down on Thursday, rescuers and media said.

Nearly 200 international and domestic flights were cancelled and authorities suspended trading on financial markets due to the extreme weather.

Two passenger boarding bridges collapsed at Taoyuan international airport, which serves Taipei and northern Taiwan, slightly damaging a China Airlines aircraft, airport officials said.

The Taiwanese carrier was forced to scrap the flight bound for Japan, affecting more than 170 passengers, they said.

The slow-moving typhoon, which killed at least 23 people in the Philippines, lost momentum and weakened into a tropical storm after it made a second landfall in the northeast around 0600 GMT, the Central Weather Bureau said.

"The public must not relax their vigilance even though Saola has weakened into a storm over the past three hours. More torrential rains are expected in many parts of the island today," an official from the bureau warned.

More than 1.0 metre (39 inches) of rainfall has fallen in parts of the island since Tuesday, according to the bureau.

Saola made first landfall near the eastern coastal city of Hualien at 3:20 am and moved back out to sea four hours later, it said.

Saola -- the first typhoon to hit the island this year -- triggered heavy rains especially in the north and east and touched off widespread mudslides, forcing the authorities to evacuate more than 1,500 people islandwide.

Television images from a police helicopter showed mudslides engulfing roads and farmland and threatening numerous households.

The defence ministry, which had ordered more than 45,000 soldiers to stand by, mobilised amphibious vehicles to rescue residents trapped by breast-deep water in northeastern Ilan county.

In Sanhsia district in the north, one man was buried by mudslides and a police officer drowned while patrolling the area, the Central Emergency Operation Centre said.

A motorcyclist died after he crashed into a toppled tree in south Taiwan's Chiayi county, it said, adding that a married couple were buried by mudslides on their farmland in Ilan.

The sixth fatality, not included in the centre's toll but reported by local media, was a woman who was hit by a falling tree while distributing newspapers by motorcycle in the southern city of Kaohsiung.

Two people were listed as missing, with one man washed away and another feared buried by a mudslide.

Many residents across the island woke to see their neighborhoods covered in ankle-deep water, with classes and work suspended everywhere except for Taidong county in the southeast.

The typhoon, packing winds of up to 108 kilometres (67 miles) an hour, left toppled trees blocking streets and hampering traffic in several places in the capital Taipei.

Rail traffic was interrupted while the high-speed railway linking Taipei to Kaohsiung cut its services to 97 trains from the scheduled 123.

More than 50,000 households were left without power and nearly 7,000 households without water, the Central Emergency Operation Centre said.

The Central Weather Bureau said the storm would continue to have an impact into the weekend, and that winds following in its wake could cause heavy rainfall in the island's south next week.

As of 1115 GMT, Saola was 60 kilometres north of Taipei, moving northwest at up to 19 kilometres an hour for the Chinese mainland.