The Observatory says the Strong Wind Signal No 3 will remain in force until at least 3pm on Wednesday.

At 10 am, Tropical Storm Mulan was about 400 kilometres southwest of Hong Kong, forecast to more northwest or west-northwest at about 18 kilometres per hour towards western Guangdong and Hainan island.

The forecaster said Mulan is now closest to Hong Kong and will depart gradually. It said it would replace the No 3 signal with No 1 when local winds show signs of weakening.

"The outer rain bands of Mulan will occasionally bring violent gusts to the territory. There will still be occasional gale force winds over the offshore waters and on high ground at first," the Observatory said.

The No 3 signal was issued at around 11.30am on Tuesday.