The Hong Kong Observatory issued the standby No 1 signal at 4.15am on Tuesday, saying two tropical cyclones may affect the city.

The cyclones are now centred within about 800 kilometres of Hong Kong.

One of them was near the southern part of Taiwan, about 640 kilometres east of Hong Kong at 6am.

The other is a tropical depression over the central part of the South China Sea, estimated to be about 770 kilometres south-southwest of the city, and is forecast to move northwest at about 10 kilometres per hour towards the vicinity of Hainan Island.

According to the present forecast, the No. 1 signal will remain in force for most of today.