Greek authorities grappling with a large forest fire, the second major blaze in as many days, evacuated four villages near Patras, Greece's third largest city.

Ninety-five firefighters, 33 trucks, four planes and helicopters have been mobilised to extinguish the fire in the Zeria region in the Peloponnese, about 210 kilometres of Athens, the firefighting service said.

The authorities have sent alerts via mobile telephones to warn residents to evacuate the villages of Achaia, Zeria, Kamara and Labiri.

Hospitals in Patras and the neighbouring city of Aigio have been put on notice to admit any injured people, while the coastguard are on standby to rescue any swimmers overcome by smoke.

The Greek press agency ANA said the highway between the Peloponnese and continental Greece had been closed.

Greece is hit by forest fires every summer, but experts have warned that global warming increases both their frequency and intensity.

Greece has been in the grip of another heat wave since Friday, with temperatures hovering between 42 and 44 degrees Celsius, weather forecasters say.

Several days ago, a fire ravaged Mount Penteli, close to the capital Athens, but caused no casualties.

It was the same area where a fire in July 2018 went on to claim 102 lives in Greece's worst-ever toll from a forest inferno.