China will see the return of more heatwaves over the next 10 days from east to west, with some cities already issuing the highest-level warning on Friday.

A sharp temperature spike is expected on Saturday, before building up into heatwaves, defined as periods of atypically hot weather of three days or more. This Saturday is the day of the "big heat" in the Chinese Almanac based on the lunar calendar.

The hot spell is expected to be similar in scope as heatwaves from July 5-17, but more regions could be hit by temperatures of 40 degrees Celsius or higher, Fu Jiaolan, chief forecaster at the National Meteorological Centre, told state media.

Some cities in Zhejiang province, home to many factories and exporters, on Friday issued red alerts, the highest in a three-tier warning system, forecasting temperatures of at least 40C in the next 24 hours.

The load on the national power grid could reach a new high this summer, with safe operation facing "severe tests", the Ministry of Emergency Management warned on Friday.

"For all of the factories in China and in Shanghai we have regulations that need to be followed," said Leo Zhang, president of chemical product maker Sika China.

"Every year we do things to make the work more comfortable, for example giving workers ice-cream when it gets too hot."

Zhejiang, as well as parts of Fujian, Guangdong, Hunan, Jiangxi and the city of Chongqing, are also at risk of forest fires in the near term, the ministry said.

The heat in China this summer has been described as extreme, boosting demand for air-conditioning by homes, offices and factories and pushing the load on power grids in more than a dozen provinces to records.

From June 1 to July 20, the Yellow River and Yangtze River basins – major centres of industry and commerce – were hit by at least 10 high-temperature days more than the norm.

As with much of the northern hemisphere, parts of China have seen record-high temperatures this summer and unusually heavy rainfall is exacerbating seasonal flooding. Climate change is considered a factor in both the warmer temperatures and heavier rainfall, as well as more frequent extreme weather.

On Thursday, a tornado blew through 11 villages in a farming region of eastern China, damaging homes, killing at least one person and injuring 25 others. Tornadoes are relatively rare in the country. (Agencies)