At least 25 people have died in China's flood-stricken central province of Henan, a dozen of them in a subway line in its capital that was drenched by what weather officials called the heaviest rains for 1,000 years, Reuters reports.

About 100,000 people have been evacuated in Zhengzhou, the capital, where rail and road transport have been disrupted, while dams and reservoirs have swelled to warning levels while thousands of troops launched a rescue effort in the province.

City authorities said more than 500 people were pulled to safety from the flooded subway, as social media images showed train commuters immersed in chest-deep waters in the dark and one station reduced to a large brown pool.

"The water reached my chest," a survivor wrote on social media. "I was really scared, but the most terrifying thing was not the water, but the diminishing air supply in the carriage."

The rain halted bus services in the city of 12 million people about 650 km (400 miles) southwest of Beijing, said a resident surnamed Guo, who had to spend the night at his office.

"That's why many people took the subway, and the tragedy happened," Guo told Reuters.

At least 25 people have died in the torrential rains that have lashed the province since last weekend, with seven missing, officials told a news conference on Wednesday.

Media said the dead included four residents of the city of Gongyi, located on the banks of the Yellow River like Zhengzhou, following the widespread collapse of homes and structures because of the rains.

More rain is forecast across Henan for the next three days, and the People's Liberation Army has sent more than 5,700 soldiers and personnel to help with search and rescue.

Schools and hospitals were marooned, and people caught in the floods flocked to shelter in libraries, cinemas and even museums.

"We've up to 200 people of all ages seeking temporary shelter," said a staffer surnamed Wang at the Zhengzhou Science and Technology Museum.

"We've provided them with instant noodles and hot water. They spent the night in a huge meeting room."

After the First Affiliated Hospital of Zhengzhou, the city's largest with more than 7,000 beds, lost all power, officials raced to find transport for about 600 critically ill patients.

The neighbouring province of Hebei issued a storm alert for some cities, including Shijiazhuang, its capital, warning of moderate to heavy rain from Wednesday.

(Video) Henan flood deaths rise to 25, largest hospital without power