Tens of millions were battling dangerously high temperatures in the United States on Saturday as record heat forecasts hung over Europe and Japan, in the latest example of the threat from global warming.

A powerful heatwave stretching from California to Texas was expected to peak as the US National Weather Service warned of an "extremely hot and dangerous weekend".

Daytime highs were forecast to range between 10 and 20 degrees Fahrenheit above normal in the west.

In Arizona, one of the hardest-hit states, residents face a daily endurance marathon against the sun.

State capital Phoenix recorded 16 straight days above 109F (43 degrees Celsius), with temperatures hitting 111F on Saturday en route to an expected 115F.

California's Death Valley, one of the hottest places on Earth, is also likely to register new peaks on Sunday, with the mercury possibly rising to 130F (54C).

Temperatures had already reached 48C by midday on Saturday and even overnight lows could exceed 38C.

Authorities have been sounding the alarm, advising people to avoid outdoor activities in the daytime and to be wary of dehydration.

At a construction site outside Houston, Texas, a 28-year-old worker who gave his name only as Juan helped complete a wall in the blazing heat.

"Just when I take a drink of water, I get dizzy, I want to vomit because of the heat," he told AFP.

The Las Vegas weather service warned that assuming high temperatures naturally come with the area's desert climate was "a DANGEROUS mindset! This heatwave is NOT typical desert heat".

Southern California is fighting numerous wildfires, including one in Riverside County that has burned more than 3,000 acres (1,214 hectares) and prompted evacuation orders.

Further north, the Canadian government reported that wildfires had burned a record-breaking 10 million hectares this year, with more damage expected as the summer drags on.

- Historic highs forecast -

In Europe, Italy faces weekend predictions of historic highs with the health ministry issuing a red alert for 16 cities including Rome, Bologna and Florence.

The weather centre warned Italians to prepare for "the most intense heatwave of the summer and also one of the most intense of all time".

The thermometer is likely to hit 40 degrees Celsius (104 degrees Fahrenheit) in Rome by Monday and even 43C on Tuesday, smashing the record of 40.5C set in August 2007.

The islands of Sicily and Sardinia could wilt under temperatures as high as 48C, the European Space Agency warned -- "potentially the hottest temperatures ever recorded in Europe".

The Athens Acropolis, one of Greece's top tourist attractions, will on Sunday close during the hottest hours for the third day running.

In France, high temperatures and resulting drought are posing a threat to the farming industry, earning Agriculture Minister Marc Fesneau criticism from climatologists on Saturday for having brushed aside conditions as "normal enough for summer".

This June was the second-hottest on record in France, according to the national weather agency, and several areas of the country have been under a heatwave alert since Tuesday.

There is little reprieve ahead for Spain, as its meteorological agency warned Saturday that a new heatwave Monday through Wednesday will bring temperatures above 40C to the Canary Islands and the southern Andalusia region.

- Killer rains -

Parts of eastern Japan are also expected to reach 38 to 39C on Sunday and Monday, with the meteorological agency warning temperatures could hit previous records.

Relentless monsoon rains have reportedly killed at least 90 people in northern India, after burning heat.

The Yamuna river running through the capital New Delhi has reached a record high, threatening low-lying neighbourhoods in the megacity of more than 20 million people.

Major flooding and landslides are common during India's monsoons, but experts say climate change is increasing their frequency and severity.

Morocco was slated for above-average temperatures this weekend with highs of 47C in some provinces -- more typical of August than July -- sparking concerns for water shortages, the meteorological service said.

- River Tigris shrinking -

Water-scarce Jordan was forced to dump 214 tonnes of water on a wildfire that broke out in the Ajloun forest in the north amid a heatwave, the army said.

In Iraq, where scorching summers are common, Wissam Abed usually cools off from Baghdad's brutal summer by swimming in the Tigris river.

But as rivers dry up, so does the age-old pastime.

With temperatures near 50C and wind whipping through the city like a hairdryer, Abed stood in the middle of the river, but the water only comes up to his waist.

"Year after year, the water situation gets worse," the 37-year-old told AFP.

While it can be difficult to attribute a particular weather event to climate change, scientists insist global warming -- linked to dependence on fossil fuels -- is behind the multiplication and intensification of heatwaves.

The EU's climate monitoring service said the world saw its hottest June on record last month.