At least 21 athletes are known to have died from exposure when a horror storm pelted a marathon route in north-west China with hail, freezing rain and gale-force winds, state media reports.

The rescue headquarters quoted by the official Xinhua News Agency said the participants suffered from physical discomfort and the sudden drop in temperature.

Some went missing in the extreme weather on Saturday afternoon, when the 100-kilometre race in the Yellow River Stone Forest tourist site in Baiyin city in Gansu province was halted.

Early on Sunday (local time), rescuers found 21 dead, including the last of the five missing following an all-night search that involved more than 700 people. The operation was made difficult by low temperatures and the complex terrain and topography.

The runners were racing on a narrow mountain path at high altitude, a reporter for state broadcaster CCTV said.

A total of 172 people had joined the race and 151 were confirmed to be safe. Some were treated for minor injuries and were stable, Xinhua said.

Baiyin Mayor Zhang Xuchen held a news conference on Sunday and profoundly apologised as the organiser of the event.

“We express deep condolences and sympathy to the families of the victims and the injured,” he said.

-with AAP