Fires raged and millions of people faced increased health risks amid a sustained spell of dangerous heat wave across three continents Wednesday.

Greece was battling wildfires that have been exacerbated by scorching weather and temperature records have been broken repeatedly in the United States, Europe and China.

Experts have said climate change, driven by the burning of fossil fuels, has played a key role in worsening destructive weather like the current extreme temperatures.

The exceptional heat has prompted safety warnings, with France on Wednesday saying temperatures of up 40 degrees Celsius (104 degrees Fahrenheit) were expected in parts of the nation's south.

Two forest fires, fanned by strong winds, were raging to the west of Athens, while another broke out on the tourist island of Rhodes – forcing locals to make painful decisions on whether to flee.

"I am not leaving. I started building this house when I was 27 years old by myself," said Dimitris Michaelous, a resident in the fire-threatened town of Pournari.

"I will stay here at least to watch it burn. I am not leaving," he added.

With a new wave of heat expected to hit Greece from Thursday, the minister called on the population to be extremely careful to not spark new fires.

Beijing broke a 23-year-old record with 27 consecutive days of temperatures above 35 degrees Celsius, forecasters said.

"At noon, it feels like the sun is roasting my legs, it feels like my skin is burning. It (Beijing summers) wasn't this hot before," said 22-year-old student Qiu Yichong.

Phoenix broke a similar record, in place for 49 years, with its 19th consecutive day of temperatures of 43.3 degrees Celsius or higher, weather officials said.

In southern France, a record 29.5 degrees Celsius was recorded in the Alpine ski resort of Alpe d'Huez, while 40.6 degrees Celsius had been recorded for the first time in Verdun in the foothills of the Pyrenees.

Heatwave on horizon

In the Canary Islands, some 400 firefighters battled a blaze that has ravaged 3,500 hectares of forest and forced 4,000 residents to evacuate, with authorities warning people to wear face masks outside due to poor air quality.

Temperatures were unforgiving in Italy and in Spain, where three regions were put under hot weather red alerts.

The Italian islands of Sardinia and Sicily have been forecast to possibly surpass a continent-wide record of 48.8 degrees Celsius recorded in Sicily in August 2021.

At Lanusei, near Sardinia's eastern coast, a children's summer camp was restricting beach visits to the early morning and forbidding sports, teacher Morgana Cucca told AFP.

In the Sardinian capital of Cagliari, pharmacist Teresa Angioni said patients were complaining of heat-related symptoms.

"They mainly buy magnesium and potassium supplements and ask us to measure their blood pressure, which is often low," Angioni said.

Many throughout Italy sought to escape by the sea, including outside Rome, where the midday heat hit 40 degrees Celsius.

"Certainly it's better at the beach, you can at least get a little wind from the sea. It's not even possible to remain in the city, too hot," said Virginia Cesario, 30, at the Focene beach near the capital.

Climate change impact

Tens of millions of Americans experienced dangerous heat levels Tuesday.

In the town of San Angelo, Texas, where temperatures were expected to reach 40 to 42 degrees Celsius, the National Weather Service said it was "running out of ways to say that it's gonna be hot out there today."

"With temperatures across the area likely topping the 105 (40.56 degrees Celsius) mark yet again, we implore you to continue to practice heat safety and try to stay cool," the agency said on Twitter.

And in Arizona, the mercury at Phoenix Sky Harbor Airport again reached 43.33 degrees Celsius on Tuesday, breaking the previous record of 18 consecutive days at or above that temperature, set in 1974.

The heat waves across Europe and the globe are "not one single phenomenon but several acting at the same time," said Robert Vautard, director of France's Pierre-Simon Laplace climate institute.

"But they are all strengthened by one factor: climate change."

The record-setting heat came as U.S. climate envoy John Kerry met with Chinese officials in Beijing, with the world's two largest polluters reviving stalled diplomacy on reducing planet-warming emissions.

Speaking Tuesday at Beijing's Great Hall of the People with China's top diplomat Wang Yi, Kerry called for "global leadership" on climate issues.