Sixteen people have been killed and dozens are missing in flash floods that have hit the northwest province of Qinghai, where a sudden massive downpour caused a river to change course, state media reported on Thursday.

Broadcaster CCTV reported the toll of dead and missing following the deluge, which comes during a summer of extreme weather on the mainland and around the world.

It said the sudden rainstorm on Wednesday night triggered a landslide that diverted a river. It said 6,200 people and over 1,500 houses in six villages had been affected, with emergency authorities describing it as a "mountain torrent" disaster.

In June, severe flooding in southern China displaced more than half a million people and caused an estimated US$250 million in damage.

Meanwhile in the country's southwest, millions of people are facing rolling electricity cuts as a crushing heatwave and drought led to power shortages that has forced factories to halt work.

The China Meteorological Administration says the country is going through its longest period of sustained high temperatures since records began in 1961, with 64 consecutive days of heat warnings in various regions starting in June. (Agencies)