Vietnam is evacuating more than half a million people and has ordered boats to stay ashore, while the southern Chinese city of Sanya closed businesses and public transport as the two countries brace for an intensifying Typhoon Kajiki.

The storm could make landfall along the southern coast of China’s southern island province of Hainan from Monday AEST, or skirt the southern coastline before heading toward Vietnam, China’s National Meteorological Centre said.

The storm, packing winds of up to 149km/h, was about 500km off Vietnam’s central coast, moving west at 20km/h, according to Vietnam’s national weather forecast agency.

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It is likely to strengthen with wind speeds as high as 170km/h, China’s weather forecaster said.

Authorities in Vietnam plan to evacuate more than 586,000 people from the central provinces of Thanh Hoa, Quang Tri, Hue and Danang, where the typhoon is forecast to make landfall early on Monday, state media reported.

Seven coastal provinces in the South-East Asian nation banned boats from leaving shore early on Sunday, Tien Phong newspaper reported.

Flag carrier Vietnam Airlines cancelled at least 22 flights to and from central cities on Sunday and Monday.

Vietjet Aviation said it was cancelling or delaying flights but did not provide details.

China’s Sanya, renowned for seafront resorts and sandy beaches, closed tourist attractions, shuttered businesses and suspended public transport.

China’s weather agency forecast heavy rainfall and strong winds in Hainan and nearby Guangdong province and Guangxi region, with areas in Hainan set to receive as much as 400mm of precipitation.

Sanya issued a red typhoon alert on Sunday - the highest in China’s colour-coded warning system - and raised its emergency response to the most severe level, according to posts on the local government’s WeChat account.

City officials convened a meeting, urging preparation for “worst-case scenarios” and stressing the need for heightened vigilance to ensure no fatalities and minimal injuries, the government said.

All classes and construction are suspended, and shopping centres, restaurants and supermarkets are closed from Sunday. Vessels have been ordered to cease operating in Sanya’s waters.

Officials said the lifting of restrictions would depend on the storm’s impact.

Sanya is one of China’s most popular holiday destinations, attracting 34 million tourist visits last year, state news agency Xinhua reported.

Since July, record rainfall has lashed China north and south in what meteorologists describe as extreme weather events linked to climate change, testing local government readiness and posing significant risk to lives and the economy.

Natural disasters including flooding and drought caused 52.15 billion yuan ($A11.26 billion) in direct economic losses in China last month, affecting millions of people and leaving 295 dead or missing, data from the Ministry of Emergency Management showed.

The Vietnamese government likened the strength of Kajiki to that of Yagi, which battered the country less than a year ago, killing about 300 people and causing property damage of $US3.3 billion.