As river embankments were breached in record downpours across Henan province, President Xi Jinping described the situation as "extremely severe" with flood control measures entering a "critical stage", state media reported Wednesday.

Around 200,000 city residents were evacuated, local government officials said, as soldiers led rescue efforts in Zhengzhou -- a city of over 10 million people -- where days of rain have inundated the streets and subway.

The rainfall in the region was the heaviest since record-keeping began 60 years ago, with Zhengzhou seeing the equivalent of a year's average rainfall in just three days.

Rainstorms submerged Zhengzhou's metro, killing 12 people and injuring five, while city officials said hundreds were rescued from the subway.

Nerve-shredding images shared on social media showed shocked passengers contending with the fast-rising waters inside a train carriage. Rescuers cut open the roof of the coach to pull people to safety, local media reported.

Others showed dramatic rescues of pedestrians in Zhengzhou from torrents gushing through the streets.

At least four were killed in the nearby Gongyi city where houses and walls have collapsed, the official Xinhua news agency said, adding that rainfall had caused multiple landslides.

Relatives outside Zhengzhou made anxious pleas on China's Weibo for information as communications to the city went down.

"Is the second floor in danger? My parents live there, but I can't get through to them on the phone," one user wrote.

"I don't know more about their situation. I'm in Tianjin and my parents are in Zhengzhou," she said, giving her surname only as Hou when contacted by AFP.

"I'm very anxious."

- Army sent -

Authorities have issued the highest warning level for Henan province as floods continue to hammer the region.

As the scale of the disaster continued to unspool on Wednesday, the Chinese army warned that a stricken dam around an hour from Zhengzhou city "could collapse at any time" after being severely damaged in torrential storms.

The regional unit of the People's Liberation Army said Tuesday that the relentless downpour had caused damage to the Yihetan dam in the nearby city of Luoyang.

On Wednesday morning the PLA said blasting operations had been carried out at the dam and troops had "successfully opened a new flood diversion opening".

These measures meant the water level had dropped and the "danger has been effectively controlled".

Soldiers have been deployed to other rivers nearby to reinforce embankments with sandbags as the floods fanned out across Henan and warnings were issued of other near breaches of dams.

"Some reservoirs had their dams burst... causing serious injury, loss of life and property damage," Xi Jinping said, according to state broadcaster CCTV.

"We have already entered the critical stage of flood control, leaders and cadres from all walks of life must... take the lead in commanding, quickly organise forces for flood protection and disaster rescue."

Annual floods during China's rainy season cause chaos and wash away roads, crops and houses.

But the threat has worsened over the decades, due in part to widespread construction of dams and levees that have cut connections between the river and adjacent lakes and disrupted floodplains that had helped absorb the summer surge.

Scientists say climate change is also worsening flooding around the world alongside other increasingly extreme weather patterns.

bys-lxc/rox/axn

Weibo