In the warning issued at 5am Friday, the department said the storm was over the Gulf of Tonkin, Guangxi, China at a latitude of 21.2 degree North and longitude of 109.2 degree East at 4am Friday.
With maximum sustained winds of about 70 knots, it was moving west slowly and was expected to make landfall over upper Vietnam later in the day.
Torrential downpours will be likely over the country especially in the Northeast and the North and may cause flash flooding and landslides in certain areas.
The warning added that the strong southwest monsoon, which is now prevailing across the Andaman Sea and the upper Gulf, would unleash more rain and isolated heavy downpours over the South and the East until Tuesday.