Vietnam has shut down airports, closed schools and begun mass evacuations as it prepares for the most powerful storm so far this year.

Typhoon Kajiki, with gusts of up to 166 kilometres per hour at sea, was around 110 kilometres off the northern part of Vietnam's central coast at 11am and was due to make landfall on Monday afternoon, the country's weather agency said.

"This is an extremely dangerous fast-moving storm," the government said in a statement on Sunday night, warning that Kajiki would bring heavy rains, flooding and landslides.

With a long coastline facing the South China Sea, Vietnam is prone to storms that are often deadly and trigger dangerous flooding and mudslides.

Kajiki is forecast to hit provinces that are less industrialised than the area struck last year by Typhoon Yagi, which killed around 300 people and caused property damage of around US$3.3 billion.

The storm is projected to move inland across Laos and northern Thailand.

The Vietnamese government said on Monday about 30,000 people had been evacuated from coastal areas.

More than 16,500 soldiers and 107,000 paramilitary personnel have been mobilised to help with the evacuation and to stand by for search and rescue, the government said in a statement.

Authorities said on Sunday that more than half a million people would be evacuated and ordered boats to remain in port.

Two airports in Thanh Hoa and Quang Binh provinces have been closed, according to the Civil Aviation Authority of Vietnam. Vietnam Airlines and Vietjet cancelled dozens of flights to and from the area on Sunday and Monday.

Kajiki skirted Hainan on Sunday as it moved towards Vietnam, forcing Sanya City on the island to close businesses and public transport.

Hainan downgraded its typhoon and emergency response alerts on Monday morning, but warned of heavy rain and isolated storms in cities in the southern part of the island.

Local media reported that many residents in Sanya, a popular holiday resort, had taken shelter from the storm in underground garages on Sunday evening. Some large trees were brought down leaving roads strewn with broken branches by Monday morning, the reports said. (Reuters)