Storms drenched parts of Guangdong province on Monday, triggering warnings of flash floods and landslides, a day after Typhoon Wipha pounded Hong Kong.

Heavy rain lashed the cities of Yangjiang, Zhanjiang and Maoming after the storm system made landfall in Taishan on Sunday evening.

The national weather forecaster said Wipha had weakened to a tropical storm upon landfall, and forecast it to skirt the coast of Guangdong and move southwesterly towards Vietnam.

Heavy rain is forecast for coastal regions in such provinces as Guangdong, Guangxi, Hainan and Fujian until Tuesday morning, with warnings of flash floods, landslides and wind hazards.

The system will move into the Gulf of Tonkin late Monday morning, over which it will gather intensity before hitting Vietnam's northern coast on Tuesday, the forecaster said.

Vietnam's Prime Minister put coastal provinces on emergency footing for Wipha, saying it could cause flooding and landslides, as airlines cancelled flights ahead of the storm's expected landfall early on Tuesday.

Wipha is expected to complicate a search for people still missing after a tourist boat capsized during a thunderstorm in Halong Bay in northern Vietnam on Saturday, killing more than 30 people.

The storm is forecast to cross Vietnam's northern coast between Quang Ninh and Ninh Binh provinces and the national weather agency has said it is carrying heavy rains that could cause dangerous floods and mudslides.

Prime Minister Pham Minh Chinh sent an urgent message to coastal provinces late on Sunday to call boats ashore, evacuate people from flood-prone areas, prepare food supplies and rescue equipment and secure communication infrastructure.

"This is a strong and fast-moving typhoon," Chinh's message said, adding that Wipha might cause flash floods and landslides in mountainous areas and floods in urban areas.

Vietnam Airlines and its affiliate, Pacific Airlines, have cancelled at least nine domestic flights on Monday, the country's flag carrier said late on Sunday, adding that more delays and reschedules are expected.

Budget airline Vietjet on Monday cancelled 12 flights, including some to South Korea. (Reuters)