China issued a fresh heatwave alert Wednesday as soaring temperatures -- some of them record highs -- were again forecast for large swathes of the nation, sparking concerns about power shortages.

The National Meteorological Centre warned that large parts of northern and central China would again be hit by "sweltering heat and very little rain", with temperatures set to hit 40 degrees Celsius (104 degrees Fahrenheit).

It set the heat alert at "orange" -- the second-highest rating on a four-colour scale.

The extreme heat, which began at the weekend in most areas, has led to hospital wards packed with people suffering from heat-related illnesses, a surge in air conditioner sales and even a plague of locusts in Inner Mongolia.

Zookeepers used giant ice blocks to keep animals cool, the Xinhua news agency reported.

In several cities including Beijing, where the temperature hit 40.6 degrees Celsius on Monday, authorities reported that overheated vehicles had caught fire, state media said.

The heatwave comes after torrential rains in June, mainly in southern parts of the nation, caused massive floods and landslides that killed at least 266 people.

The soaring temperatures have led to fears of power shortages in several provinces due to high demand for air conditioning, the state-run Global Times reported.

In the southern city of Guangzhou, authorities had opened nearly 500 shelters equipped with air conditioning units and water for people to escape the heat, the report said.

Water demand has also soared along with the high temperatures.

In Beijing on Monday, nearly three million cubic metres of water was pumped into the capital, the largest single-day usage since tap water was brought into operation in 1910, the China Daily reported.

Authorities in the capital said they would double a "high temperature" subsidy for people working outside in the heat, or whose workplaces were hotter than 33 degrees Celsius.

Those who work outside will now get an extra 120 yuan (18 dollars) a month, while people in hot indoor places will be paid 90 yuan, the report said.