At least 25 people were killed and as many as 31 others went missing as a slow-moving tropical storm dumped enormous amounts of rain across the Philippines' main island, authorities said Wednesday.

Nock-ten, named after a Laotian bird, was expected to cause more damage in the mountainous northern areas of Luzon island on Wednesday night, while also bringing heavy rain to Manila, civil defence chief Benito Ramos told AFP.

"We are closely watching northern Luzon. There have already been landslides there but no casualties (there) so far," said Ramos, chief of the National Disaster Risk Reduction and Management Council.

Nock-ten, which struck the eastern Philippines on Tuesday, had already killed 25 people, with many of the victims caught in floods or consumed by landslides, Ramos said.

Most of the missing were fishermen or sailors lost at sea, he added.

More than 645,000 people were also forced to flee their flooded homes, particularly in the southeast of Luzon where Nock-ten first hit land.

He said the government was waiting for the skies to clear and the seas to calm before sending emergency supplies by air and water to those provinces.

"We can't use the army trucks because the roads are flooded," he said.

The council said dozens of flights had been cancelled because of the storm.

Nock-ten had initially been expected to pass much closer to Manila, the nation's sprawling capital of 12 million people, and schools were closed across the city on Wednesday as it prepared for heavy rains.

But Ramos said Nock-ten was following an erratic course and the worst of its fury had spared the densely populated areas of Manila and its suburbs.

The storm, now packing maximum winds of 100 kilometres (60 miles) per hour, was moving northwest at 17 kilometres per hour and was expected to be out of the country by late Wednesday, the government weather station said.

Nevertheless, Ramos warned that all areas within 500 kilometres of the eye of the storm, including Manila, should expect rain until it moved out to the South China Sea.

The new course of the storm still endangers about four million people in mountainous areas of northern Luzon which are prone to landslides and flash floods during heavy rains, authorities said.

Philippine President Benigno Aquino expressed frustration at people who continued to move to these vulnerable areas, a common problem in a country struggling with a booming population and chaotic urban planning.

"(The government) put up markers saying it is dangerous for people to build houses there because they are prone to landslides. The problem is people don't pay attention and still build in these areas," Aquino told reporters.

An average of 20 storms and typhoons, some of them deadly, hit the Philippines every year. Storms killed 48 people on Luzon in May and June.

Unusually heavy rains also killed 42 people last month in the country's south, an area that is normally spared typhoons and storms.

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