The Hong Kong Observatory (HKO) issued Standby Signal No. 1 at 7.40am on Thursday morning as a tropical depression heads towards Hainan Island.

The observatory said the warning signal will remain in force for the day, and it will assess the need to upgrade to a No. 3 signal on Friday depending on the change of the system's intensity, the distance of its strong winds from the pearl river estuary and local wind conditions.

The HKO said the system is expected to intensify gradually as it moves west-northwest. The depression is forecast to maintain a distance of 500 kilometres or more from Hong Kong for the day.

With the system moving closer to the southern Chinese coast, forecasters say local winds will strengthen gradually tomorrow and the weather will become unsettled, with more squally showers and brief bursts of gale force winds on high ground expected.

The observatory urged members of the public to stay away from the shoreline and refrain from water sports.