A resident walks past tourist boats in Pitan, in New Taipei City, as Typhoon Soudelor approaches eastern Taiwan on Friday.

AFP/Taipei

An eight-year-old girl died after being swept out to sea off Taiwan as Typhoon Soudelor bore down on the island, forcing thousands to flee and troops to be placed on standby, officials said on Friday.

Billed as the biggest typhoon of the year earlier in the week, Soudelor has since weakened but authorities warned it may strengthen once more before making landfall.

The young girl became the first casualty of the impending storm after she was swept out to sea in eastern Yilan county on Thursday, authorities said.

Another child was also missing in the same incident, while a 38-year-old woman and another girl survived.

"The girl who died and the one who went missing were swept away by strong waves," a spokesman for the National Fire Agency told AFP.

More than 2,000 people have already been relocated from Taiwan's outlying islands, popular with tourists, and troops were preparing to help more residents move from their homes into shelters as rains began to fall.

Riverside aboriginal communities in New Taipei City will be evacuated on Friday evening when downpours are expected to become much heavier.

The storm is set to make a direct hit on the east coast, though much of the island will be lashed by torrential rain.

By Friday afternoon 140 millimetres had already fallen near Taoyuan City in the northwest.

In two coastal counties in east Taiwan - Yilan and Hualien - offices and schools shut down on Friday morning with more counties, mainly in the north and east, expected to follow suit later in the day.

More than 40 international flights out of Taiwan were cancelled and ferry services to outlying islands were suspended.

Packing maximum wind speeds of 173 kilometres per hour near its centre, Soudelor was 370 kilometres southeast of eastern Hualien county on Friday afternoon.

It was due to make landfall on the central east coast late Friday or early Saturday morning.