Taiwan put tens of thousands of troops on standby and shut schools and offices in an offshore island Wednesday as weather forecasters warned of torrential rain from Tropical Storm Kong-Rey.

"Due to the storm, the mountainous areas in the centre and south are forecast to have a large amount of rainfall from Wednesday afternoon to Thursday," the Central Weather Bureau said in a statement.

As the storm also brought fierce winds, the government of Orchid Island, a popular scenic spot off the southeastern coast, decided to close schools and offices.

Flights and ferries to and from the small island were also suspended, the local government said.

Although the storm was not likely to make landfall, the military stepped up its preparations and ordered more than 47,000 troops to be on standby.

As of 0630 GMT, the storm was 190 kilometres (120 miles) east of the island's southern tip, the bureau said.

Packing gusts of 119 kilometres per hour, up from 109 kilometres recorded the previous day, Kong-Rey was moving north at 15 kilometres per hour, it said.

Tropical Storm Trami hit Taiwan last week, forcing authorities to close offices and cancel nearly 200 international flights.