The tropical depression strengthened from a low-pressure system over the Gulf of Tonkin on Friday morning and was centred southeast of Bach Long Vi Island at 1 pm, according to the National Centre for Hydro-Meteorological Forecasting.
It is forecast to move east-southeast at about 5 kph and reach an area north of Hainan Island, China, by Saturday afternoon, before weakening into a low-pressure area by Sunday afternoon.
Authorities have been told to monitor vessels, particularly fishing boats and tourist craft, issue timely warnings and prepare rescue teams and equipment. State media have also been asked to provide regular updates.