Large swathes of China's central Henan province were under water on Wednesday, with its capital Zhengzhou hardest-hit after being drenched by what weather forecasters said was the heaviest rain in 1,000 years.

In Zhengzhou, a city of over 12 million on the banks of the Yellow River, 12 people have died so far amid the floods, and about 100,000 people have been evacuated to safe zones, the official Xinhua agency reported, citing the local government.

The lives of millions of people in Henan have been upended since the weekend in an unusually active rainy season that has led to the rapid rise of a number of rivers in the vast Yellow River basin.

Many train services across Henan, a major logistics hub in central China, have been suspended. Many highways have also been closed and flights delayed or cancelled.

Roads in a dozen populous cities have been flooded. Videos on social media showed residents, their arms interlocked, wading across streets in fast-moving water. One video showed an adult and a child being swept away on a major artery choked with half-submerged vehicles.

From the evening of Saturday until late Tuesday, 617.1 millimetres (mm) of rain had drenched Zhengzhou - almost on par with the city's annual average of 640.8 mm.

The level of rainfall in Zhengzhou witnessed over the three days was one seen only "once in a thousand years", local media cited meteorologists as saying.

Dramatic video shared on social media on Tuesday showed commuters chest-deep in murky floodwaters on a lightless subway train and an underground station turned into a large, churning pool. read more

"The water outside the cabin was rising, and it was leaking in from the door," an internet user wrote on social media after surviving the ordeal.

"The water reached my chest. I was really scared, but the most terrifying thing was not the water, but the increasingly diminishing air supply in the cabin."

Dozens of reservoirs and dams have also breached warning levels.

Overnight, local authorities said the rainfall had caused a 20-metre breach in the Yihetan dam in the city of Luoyang west of Zhengzhou, and that the dam "could collapse at any time".

In Zhengzhou, the local flood control headquarters said the city's Guojiazui reservoir had been breached.

More rain

"Flood prevention efforts have become very difficult," President Xi Jinping said on Wednesday, addressing the situation in a statement broadcast by state television.

On Wednesday, Zhengzhou's transportation system remained paralysed, with schools and hospitals cut off by severe waterlogging, according to state media.

Some children have been trapped in their kindergartens since Tuesday.

The First Affiliated Hospital of Zhengzhou - a large treatment centre with over 7,000 beds - has lost all power, and even backup supplies were down, the People's Daily reported on Wednesday.

The hospital was racing to find transport to relocate about 600 critically ill patients, according to the report.

Heavy rain is forecast to persist through Wednesday, though precipitation will continue across the province for the next three days.