Four people were killed and nine others injured in a flash flood in Sichuan province on Saturday, local authorities said.

Footage published by mainland media showed water rising rapidly in a river on the outskirts of Pengzhou city.

Tourists who had been playing in the initially shallow water could be seen running for safety and clambering over rocks as the water rushed towards them, but some were unable to reach the river bank in time.

In a video posted online by the state-owned Beijing Youth Daily, a woman stranded on a boulder in the middle of the river appeared to lose her footing and was swept away by the current.

"As of 7.30pm, the mountain flood has killed four people, severely injured three and lightly injured six others," Pengzhou emergency response authorities said in a statement.

The flood comes during a summer of extreme weather in various parts of China, with multiple cities including Shanghai recording their hottest days ever during a heatwave in July.

Scientists say extreme weather across the world has become more frequent due to climate change, and will likely grow more intense as global temperatures rise.

China's national observatory on Saturday issued a red alert for high temperatures as the mercury was expected to soar past 40 degrees Celsius across swathes of the country this weekend.

Severe flooding in the south of the country in June displaced more than half a million people. (AFP)