The toll of dead and missing is likely to rise as officials assess the full impact of the storm, the strongest to hit the country this year, said Emilia Tadeo of the civil defence damage report section.

"After the rains have subsided, that is only when we find the additional casualties and damages, when the local responders submit them to us," Tadeo said.

Five people were killed by landslides and two others drowned while six people vanished at sea or were swept away by overflowing rivers as Nanmadol brought heavy rains to northern Luzon, the civil defence office said.

Almost 20,000 people were forced to flee their homes due to the risk of floods and landslides in the mountainous area.

Eight bridges were destroyed and 20 major roads left impassable after Nanmadol hit the country with gusts of up to 230km/h, the civil defence office added.

Nanmadol had weakened after clipping the northern edge of Luzon but alerts remained in force on Sunday as the typhoon edged towards Taiwan.

The storm, still packing gusts of 170km/h, was charted by the government weather station on August 28 moving towards the eastern coast of Taiwan at about 7km/h.

Nanmadol is forecast to hit Taiwan by August 31, the meteorologists said, warning that the storm could gain strength as crossed the sea.