Vietnam announced plans to evacuate more than half a million people and ordered boats to stay ashore, while the southern Chinese city of Sanya closed businesses and public transport on Sunday as both braced for an intensifying Typhoon Kajiki.
The storm is forecast to "brush past" the southern coast of China's island province of Hainan from Sunday evening before heading for Vietnam, China's National Meteorological Center said.
As of 0900 GMT, the storm had gained strength, with winds reaching 166 kph (103 mph), according to Vietnam's national weather forecast agency.
It is likely to strengthen further with wind speeds as high as 180 kph, China's weather forecaster said.
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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.
People in the projected path should not go outdoors after 1400 GMT on Sunday, and soldiers are standing by to help, the government said.
Seven coastal provinces in Vietnam banned boats from leaving shore early on Sunday, Tien Phong newspaper reported.
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.
'WORST-CASE SCENARIOS'
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 320 mm (12.6 inches) of rain from Sunday to Monday.
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Sanya issued a red typhoon alert on Sunday morning - 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 on Saturday evening, urging preparations for "worst-case scenarios", the government said.
All classes and construction were suspended, and shopping centres, restaurants and supermarkets were closed from Sunday. Vessels have been ordered to stop operating in Sanya's waters.
Since July, record rainfall has lashed China's north and south in what meteorologists have described as extreme weather linked to climate change.
Natural disasters including flooding and drought caused 52.15 billion yuan ($7.28 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 $3.3 billion of property damage.
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