Tens of thousands were left stuck at home or fleeing floodwaters that reached the tops of cars and rooftops in areas of Thai Nguyen city, about 80 kilometres (50 miles) north of the capital Hanoi.

The environment ministry said eight people were killed in flash floods and landslides in Vietnam's mountainous north since Monday, and five others were missing.

By Wednesday morning, the weather bureau said the level of the Cau river, running across Thai Nguyen city, was more than a metre higher than the previous record level of 28.81 metres (94.5 feet) -- when Typhoon Yagi devastated the country in September last year.

Overnight Tuesday and Wednesday morning, social media users posted pleas for help as their relatives and friends were left stranded with no electricity and few provisions in the provinces of Thai Nguyen, Cao Bang and Lang Son.

"Our ground floor (in Thai Nguyen province) was totally flooded. My parents and five kids were stuck, with not enough foood and water. No communication since late Tuesday. They need urgent help," Thoan Vu posted online alongside hundreds of similar pleas.

The floods followed heavy rain from Typhoon Matmo, which weakened on Monday while approaching Vietnam but hit the north hard.

Matmo landed only a week after Typhoon Bualoi triggered widespread flooding, killing at least 56 people and causing economic losses estimated at more than $710 million.

"I have never witnessed such a terrible flood since I was born 60 years ago," Nguyen Van Nguyen told AFP from his three-storey house in Thai Nguyen province.

"There has never been flooding here in my street but now my ground floor is all submerged."

The military said it used two helicopters to drop four tonnes of water, instant noodles, dry cake, milk and lifejackets to people in flooded parts of Lang Son province bordering China.

Human-driven climate change is turbocharging extreme weather events like typhoons, making them ever more deadly and destructive.