Tourists stand for a photo next to a digital display of an unofficial heat reading at Furnace Creek Visitor Centre during a heatwave in Death Valley National Park, California, on Sunday. AFP

Health authorities have sounded alarms from North America to Europe and Asia, urging people to stay hydrated and shelter from the burning sun, in a stark reminder of the effects of global warming.

From North America to Europe and Asia, people gulped water and sought shelter from the sweltering heat, with the mercury expected to reach new highs in several places in the next few days.

'OPPRESSIVE US HEAT'

In western and southern states in the US, which are used to high temperatures, more than 80 million people were under advisories as a "widespread and oppressive" heatwave roasted the region.

California's Death Valley, often among the hottest places on Earth, reached a near-record 52˚C on Sunday afternoon. In Arizona, the state capital Phoenix recorded its 17th straight day above 43˚C, as temperatures hit 45˚C on Sunday afternoon.

"We're used to 110, 112 (degrees Fahrenheit) ... But not the streaks," Nancy Leonard, a 64-year-old retiree from the nearby suburb of Peoria, told AFP. "You just have to adapt." Southern California was fighting numerous wildfires, including one in Riverside County that has burned more than 7,500 acres and prompted evacuation orders.

Europe, the globe's fastest-warming continent, was bracing for its hottest-ever temperature this week on Italy's islands of Sicily and Sardinia, where a high of 48˚C is predicted, according to the European Space Agency.

"We're from Texas and it's really hot there, we thought we would escape the heat but it's even hotter here," Colman Peavy, 30, said as he sipped a capuccino at an outside terrasse in central Rome with his wife Ana at the start of a two-week Italian vacation.

With June already having been the world's hottest on record, according to the EU weather monitoring service, Mother Nature seemed intent on July not falling far behind.

Near Athens, a forest fire flared in strong winds by the popular beach town of Loutraki where the mayor said holiday camps for youngsters had come under threat.

"We have saved 1,200 children who were in the holiday camps," said mayor Giorgos Gkionis. Greek police arrested a foreign man suspected of starting another of the ongoing wildfires, in Kouvaras, some 50 kilometres southeast of Athens, fire service spokesman Yannis Artopios said.

In Cyprus, where temperatures are expected to remain above 40˚C through Thursday, a 90-year-old man died as a result of heatstroke and three other seniors were hospitalised, health officials said.

In Japan, heatstroke alerts were issued in 32 out of the country's 47 prefectures, mainly in central and southwestern regions. At least 60 people in Japan were treated for heatstroke, local media reported, including 51 who were taken to hospital in Tokyo.

The heat was enough for at least one man to dispense with social mortification in Hamamatsu city.

"It's honestly unbearable without a parasol, although I have to admit it is a bit embarrassing," he told national broadcaster NHK of the umbrella in his hand.

HISTORIC HIGHS FORECAST

In Europe, Italians were warned to prepare for "the most intense heatwave of the summer and also one of the most intense of all time," with the health ministry sounding a red alert for 16 cities including Rome, Bologna and Florence.

Temperatures were due to hit 42˚C-43˚C in Rome on Tuesday, smashing the record of 40.5˚C set in August 2007. Nevertheless, visitors thronged to tourist hot spots like the Colosseum and the Vatican.

"I'm from South Africa. We're used to this heat," said Jacob Vreunissen, 60, a civil engineer from Cape Town. "You have to drink lots of water, obviously wear your hat and that's about it."

Greece saw a respite on Monday, as temperatures eased a bit and the Acropolis in Athens resumed its regular opening hours after shutting for a few hours during the previous three days.

But a new heatwave was expected from Thursday and authorities on Monday ordered several seaside resorts evacuated as a precaution after a wildfire broke out in Kouvaras, 50 kilometres east of Athens.

"It's a difficult fire, the winds are really strong," said firefighters spokesman Yannis Artopios as seven water planes, four helicopters and 150 firefighters battled the blaze.

In Romania, temperatures are expected to reach 39˚C on Monday across most of the country.

Little reprieve is forecast for Spain, where meteorologists warned that "abnormally high" temperatures on Monday, including up to 44˚C in the southern Andalusia region in what would be a new regional record.