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

Footage published by Chinese media showed water rising rapidly in a river on the outskirts of Sichuan province's 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.

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

"As of 7.30 pm, 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 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 has issued a red alert for high temperatures as the mercury is expected to soar past 40 degrees Celsius (104 Fahrenheit) across swathes of the country this weekend, state news agency Xinhua reported on Saturday.

Severe flooding in southern China in June displaced more than half a million people and caused an estimated $250 million in economic damage.