Soldiers are leading the rescue in the city of more than 10 million people in Henan province, which has been hit by record rains which inundated the streets and the subway.

The city has experienced "a series of rare and heavy rainstorms, causing water to accumulate in Zhengzhou metro", city officials said in a Weibo post

The post said 12 people died and five were injured, while hundreds were rescued from the subway.

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

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

Relatives outside the city 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."

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.

Late Tuesday the regional unit of the People's Liberation Army warned that the relentless downpour had caused a 20-metre breach in the Yihetan dam in Luoyang -- a city of around seven million people -- with the risk that it "may collapse at any time".

The PLA's Central Theater Command said it had sent soldiers to carry out an emergency response including blasting and flood diversion.

"On July 20, a 20-metre breach occurred at the Yihetan dam.... the riverbank was severely damaged and the dam may collapse at any time," it said in the statement.

Soldiers have been deployed to other rivers nearby to reinforce embankments with sandbags as the floods fanned out across Henan.

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.

According to the authorities, 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.