The Observatory issued the Standby Signal No 1 at 10.20pm on Friday, as a tropical depression was centred within 800 kilometres of Hong Kong.

It said it strengthened from an area of low pressure near Luzon, and is forecast to intensify progressively and move in the general direction of the Guangdong coast.

Forecasters said the No 1 signal will remain in force for much of Saturday.

"Depending on the intensity of the tropical cyclone, the distance of its strong winds from Hong Kong and the change in local wind conditions, the Observatory will assess the need for issuing higher Tropical Cyclone Warning Signals on Sunday," it said.

Under the influence of the storm, forecasters expect that local winds will strengthen gradually later on Sunday and the weather will become unsettled.