Hundreds fled in central Italy as gas tanks exploded in a forest fire near the Tuscan town of Lucca. Similar numbers fled in Greece as a blaze fuelled by strong winds raged in mountains north of Athens.

Greek authorities said later in the day that the blaze had been tamed.

A brutal heatwave with spikes well above 40C settled over southern Europe last week, part of a global pattern of rising temperatures, widely attributed by scientists and climatologists to human activity.

It is forecast to dump searing heat on much of China into late August. It is also expected to expose about 100 million Americans to temperatures above 38C on Wednesday and set records in Texas, Louisiana and Arkansas.

While the record heat last week around parts of the Mediterranean has eased, mercury readings have begun heading up again in Portugal, Spain and Italy.

Armando Silva, Civil Protection commander in Portugal's northern region, said rising temperatures and strong winds would make it harder to fight the country's largest wildfire centred on the municipality of Murca.

It has burned 10,000-12,000 hectares since Sunday and about 800 firefighters and six water planes have been deployed to tackle it.

In Spain, where emergency crews were fighting fires in five regions, national weather service AEMET also forecast higher temperatures.

Wildfires burned in several areas of Italy, including one that threatened to leave part of the northeastern city of Trieste without power and water, and 14 metropolitan areas including Rome, Milan and Florence were due to be put on the country's highest heatwave alert on Thursday.

Forecasters there said temperatures were expected to hit 40C across the north and centre this week.

That mark was topped in Britain for the first time on Tuesday. At least 13 people died while swimming to cool off.

The chief of science and technology at Britain's Met Office, Stephen Belcher, said that unless emissions were reduced, the country might experience similar heatwaves every three years.

Treasury minister Simon Clarke said Tuesday's "remarkable, unprecedented" record served as "a reminder ... of the importance of tackling climate change".

British engineers raced on Wednesday to fix train tracks that buckled in the heat after firefighters worked through the night to damp down wildfires.

The shift in climate is leading to more wildfires and will force France and the European Union to take "structural decisions...in the years to come", President Emmanuel Macron said on Wednesday.

In southern Europe, far larger wildfires continued to rage.

In Italy, emergency crews in Tuscany battled the Lucca wildfire, which forced about 500 people to leave as flames reached villages overnight and caused liquefied gas tanks to explode, the region's governor, Eugenio Giani, tweeted.

Another fire close to the border with Croatia and Slovenia forced state-owned shipbuilder Fincantieri to close down its plant in the port city of Monfalcone.

In Greece, thick smoke darkened the sky over Mount Penteli, north of Athens, where close to 500 firefighters managed to stem the spread of a wildfire that forced the evacuation of nine settlements and a hospital.

In France, where firefighters in the southwestern Gironde region have been battling since July 12 to contain huge forest fires, Agriculture Minister Marc Fesneau said more money needed to be invested to tackle such threats.

Australian Associated Press