Large swathes of southern and eastern Europe were placed on heatwave red alert on Tuesday and the World Meteorological Organization warned of an increased risk of deaths as extreme weather gripped the continent, Asia and the United States.

The Mediterranean island of Sardinia could see highs of more than 47 Celsius (116 Fahrenheit) and forecasters said temperatures could hit 40 degrees in several Italian cities, including 42-43 degrees in the Lazio region that includes Rome.

With baking temperatures hitting Europe during the peak summer tourist season, the World Meteorological Organization (WMO) said the heatwave in the northern hemisphere was set to intensify. An estimated 61,000 people may have died in heatwaves last year in Europe alone.

The EU's emergency response coordination centre issued red alerts for high temperatures for most of Italy, northeastern Spain, Croatia, Serbia, southern Bosnia and Herzegovina, and Montenegro.

Heatwaves this summer, which saw temperatures climb to 53 degrees in California's Death Valley and over 52 degrees in China's northwest, have coincided with wildfires from Greece to the Swiss Alps and deadly flooding in India and South Korea.

They have added fresh urgency to talks this week between the United States and China, the world's top greenhouse gas polluters.

U.S. climate envoy John Kerry met Chinese officials in Beijing and expressed hope that climate cooperation could redefine troubled ties between the two powers.

Chinese President Xi Jinping stressed Beijing's commitment to carbon neutrality and a carbon peak was certain but that it would not be influenced by others.

"Temperatures in North America, Asia, and across North Africa and the Mediterranean will be above 40°C for a prolonged number of days this week as the heatwave intensifies," the WMO said.

Overnight minimum temperatures were also expected to reach new highs, the WMO said, creating the risk of increased cases of heart attacks and deaths.

"Whilst most of the attention focuses on daytime maximum temperatures, it is the overnight temperatures which have the biggest health risks, especially for vulnerable populations," it said.

The heat in Europe could also prompt a lasting shift in tourist habits, with more people choosing cooler destinations or travelling in spring or autumn, tourism organisations predicted.

In Spain, in the final throes of an election campaign ahead of Sunday's vote, politicians have adapted to the blistering heat by changing the location and timings of their rallies or limiting outdoor campaigning and switching to online events.

Hottest summers on record

Scientists have long warned that climate change, caused by greenhouse gas emissions mainly from burning fossil fuels, will make heatwaves more frequent, severe and deadly. They say governments need to take drastic action to reduce emissions.

The European Union's Copernicus Climate Change Service says 2022 and 2021 were the continent's hottest summers on record. Europe's highest recorded temperature of 48.8C was registered in Sicily two years ago.

In Italy, tourists have tried to keep cool by splashing in Rome's fountains and standing under giant fans set up outside the Colosseum. Some were forced to queue for taxis for more than an hour in the heat outside the central railway station in Rome due to the capital's chronic shortage of cabs.

The health ministry issued red weather alerts for 20 of the country's 27 main cities on Tuesday, with the number expected to rise to 23 on Wednesday.

"It is not ruled out that we will exceed 47 degrees, and there could be some places in Sulcis and Campidano (in southern Sardinia) that could make us record an even higher value," said Carlo Spanu, from the Italian air force's weather service.

"Our historical record (in Sardinia) is 47.7 degrees. Nothing prevents us from exceeding or equalling it," he said.

The heat has prompted some travellers to go home early. Anita Elshoy and her husband returned to Norway from their vacation spot of Vasanello, a village north of Rome, a week earlier than planned.

"(I) got a lot of pain in the head, legs and (my) fingers swelled up and I became more and more dizzy," Elshoy said of her heat-related symptoms.

Areas of the northeastern Spanish regions of Catalonia and Aragon, and the Mediterranean island of Mallorca were on alert for temperatures over 40C on Tuesday. The Catalonian Weather Service said the mercury reached 45C at the Boadella reservoir near the village of Darnius, the highest temperature ever recorded in the region.

Monday night's temperature did not fall below 25C in many parts of the Mediterranean coast and the interior of the Iberian peninsula, national weather agency AEMET said.

"I kind of struggled, but I have a ceiling fan and it helped me, I was able to sleep but it was hard," said Mercedes, a 60-year-old secretary in Madrid.

In Greece, authorities told citizens close to a forest fire in Dervenochoria, north of Athens, to shut doors and windows as smoke approached.

Standing in his burned-out house in Ano Lagonissi that had been his home for 32 years, Giorgos Nikolau, 89, described how he fled the fire with just the swimming trunks and shirt he was wearing.

"I have nothing else, I don't even have other shoes. Nothing. I am finished," he said.

In China, trees fell on vehicles, a whale washed ashore and a freezer full of ice cream floated off in floods as Typhoon Talim made its way across southern provinces on Tuesday, the first to make landfall in the country this year.