Heavy rains initiated by a typhoon have triggered landslides in southern China that have buried homes and vehicles and killed dozens of people, as the number of dead or missing from recent flooding in the country surged past 200.

Heavy rains killed 21 construction workers as flooding spread to northwest China, state media reported on Wednesday.

Sudden rainstorms and hail battered Haixi in Qinghai province, "washing away" the workers on Tuesday evening, the official Xinhua news agency said.

"The workers were repairing a construction site at the time when the tragedy happened," it added. A search for three missing people was under way.

Authorities said at least 85 people in county's northwest were confirmed dead, 105 missing, and some 3.74 million people had been affected by the severe weather.

The worst-affected province was Liaoning, where Xinhua quoted Guo Shouying, 54, as saying: "Floodwater gushed out of the embankments and my mother was swept away.

"The neighbours heard her desperate calls for help, but the flood was so huge that no one dared to swim into the water to rescue her."

Another 49 people have perished in Hunan, central China, and in the southern provinces of Guangdong and Guangxi, where vehicles were covered in mud and rocks along a mountain highway, the ministry of civil affairs announced.

The flooding has destroyed homes and sent rivers of mud flowing into city streets. Residents scrambled to escape the rising waters, using makeshift rafts and any implements they could find, including inflatable paddling pools, to flee. Many had to climb trees to avoid being swept away.

Chinese Premier Li Keqiang called for "persistent efforts" to save victims from what Xinhua described as "the worst floods in decades".

Nearly 3,000 military personnel were mobilised to help with the relief efforts.

Rains brought by last week's Typhoon Utor have caused severe flooding across Hunan, Guangxi and neighboring Guangdong province. In Guangdong, 22 people have died and eight were missing in flooding since Friday.

By Tuesday morning, a total of 105 people were reported dead and 115 missing in the extreme south and northeast.

In the deadliest incident, torrential rains over the last week caused the Nei River in northeastern Liaoning province to overflow near the city of Fushun, sweeping away homes, roads and utilities and leaving 54 people dead and 97 missing.

Flooding hits China each summer, but heavy rains have brought greater than usual levels of destruction in some areas.