Cities in southern China suspended schools and cancelled some flights on Thursday, as tropical storm Yagi strengthened into a super typhoon and barreled straight for the holiday island province of Hainan.

Yagi triggered floods and landslides on the main Philippine island of Luzon this week, leaving at least 13 dead, according to official figures.

The tropical storm intensified into a super typhoon as it tracked west across the South China Sea, China's Xinhua news agency said, packing winds of up to 209 kilometres (130 miles) per hour as it headed for Hainan.

"Hainan upgraded its emergency response to Yagi to the highest level at 11:30 am Thursday, according to the provincial disaster management authority," Xinhua said.

The typhoon is expected to make landfall on Friday afternoon in either Hainan or neighbouring Guangdong.

Work, school and local transport services were suspended from Thursday noon (0400 GMT) in Haikou, Hainan's capital.

In Hong Kong, which Yagi is expected to bypass, the weather observatory said it will issue the city's third-highest typhoon warning at 6:20 pm (1020 GMT), limiting public transport across the finance hub.

Classes at kindergarten and special schools in Hong Kong were suspended Thursday, while budget airline Hong Kong Express said six flights have been rescheduled.

Greater Bay Airlines said they would cancel four flights and reschedule two more due to the weather.

"Yagi will remain at super typhoon intensity and skirt around 300 kilometres (186 miles) to the southwest of Hong Kong tomorrow morning," the observatory said.

"Gale winds associated with Yagi are expected to affect the vicinity of the Pearl River Estuary tonight and tomorrow morning."

After southern China, Yagi will move towards Vietnam, on course to hit the northern and north-central regions around the famed UNESCO heritage site Halong Bay.

Vietnam's meteorological agency issued a storm warning on Thursday, and the government mobilised more than 2,700 military personnel for storm response.

Coastal provinces are planning a sailing ban on Friday, while Vietnam's mountainous provinces were ordered to prepare rescue vehicles.