The Meteorological Department warned of heavy rain and flooding in many parts of the country on Tuesday and Wednesday due to the influence of tropical storm Kajiki.
Phuwiang Prakhammin, director-general of the department, said that the storm passed over Hue, central Vietnam, at about 4am and was moving slowly in a southwesterly direction.
It would cause heavy rain, especially in the Northeast and the East, and flash floods and runoffs were possible, he said.
Rain is also forecast in the North, the Central Plains and the South.
Mr Phuwiang warned that small boats should be kept ashore for the time being due to high waves.