A punishing heatwave gripped southern Europe on Monday, prompting authorities to issue health and wildfire warnings as temperatures are expected to soar again.

France, Italy, Portugal and Spain have been sweltering for several days as the mercury climbed to 44 degrees Celsius in some places.

"This is unprecedented," Agner Pannier-Runacher, France's ecology transition minister said as a record 84 of the nation's 96 mainland departments were placed on the second-highest orange heat alert.

Only a small sliver of the country in the northwest was not sweltering, according to the Meteo France weather service, which said the heatwave was due to peak on Tuesday and Wednesday.

The summer's first major heatwave has seen authorities in the countries along the Mediterranean's northern coast urging people to seek shelter.

Ambulances stood on standby near tourist hotspots as experts warned that such heatwaves, intensified by climate change, would become more frequent.

Firefighters were on standby after blazes broke out on Sunday in France and Turkey, fed by the heat and strong winds.

Already last week, Greek firefighters had to battle a forest blaze on the coast south of Athens that forced some evacuations.

Spain's weather service AEMET said temperatures in Extremadura and Andalusia, in the south and southwest, had reached up to 44 degrees on Sunday.

And in Italy, 21 cities across the length of the country were on high alert for extreme heat, including Milan, Naples, Venice, Florence, Rome and Catania. (AFP)