A manicured hand touches the train carriage window as a brown swirl of floodwater squeezes up against the tunnel outside -- one of many scenes of desperation from an underground tragedy shared Wednesday across a stunned Chinese social media. 

At least twelve died and five others were injured in the subway flood, according to city authorities, as water coursed below ground on Tuesday evening in Zhengzhou in central China's Henan province.

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Social media platform Weibo and local media outlets carried fragments of the horror -- video posts seemingly made as a final testimony -- of chest-high and rising water inside carriages as lights went out on the city's 'Line Five' during the commuter rush hour.

Videos showed platforms submerged by a fast-flowing muddy deluge, while inside commuters - some bemused, others terrified - stood as the water rose ominously around them, knocking the power out and forcing parents to hold up their children.

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One video showed a woman's hand with painted nails, gently pushing at the carriage window, a stirring sign of incredulity at the surging water level outside - a moment of dread before the inevitable breach of the carriage doors.

"Water was leaking from the cracks in the door, more and more of it, all of us who could, stood on the subway seats," another woman said on Weibo.

She was making her way home around 5 p.m. on Tuesday when her train halted between two stations close to the city centre.

Another user on Weibo recounted being forced back into a carriage after failed attempts to evacuate.

"In the half-hour that, followed the water level became higher and higher inside the train, from our ankles to our knees to our necks."

"The power went out. Half an hour later it got hard to breathe."

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Survivors said parents lifted their children above the torrent as dread gripped the carriages.

Suddenly the glass was smashed by rescuers, who state media said also cut into the stricken carriages from above to pull the passengers out to safety.

A male survivor named Zhang told state broadcaster CCTV: "My shirt, my backpack -- everything I could throw away, I threw away. The people around me clutched onto the railings as about a dozen of us were climbing (out of the tunnel)."

Heavy rainstorms that have battered Zhengzhou since Saturday were blamed for the calamity.

Days of record rains poured down on the city of 10 million and its surroundings, but nothing prepared residents for what was about to happen.

Social media blew up with messages from panicked relatives of residents in Zhengzhou desperate to reach home as communications 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.

"Please tell me. Thank you. I'm very anxious."

Severe rainstorms kill 16
At least 16 people have died after torrential rains caused landslides and flooded a city in central China, with shocking images showing passengers struggling against chest-high water inside a train carriage.

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.