A fast-moving wildfire fuelled by gale-force winds raged unabated closer to Athens on Monday, burning trees, houses and cars and forcing evacuations of hospitals and at least 11 towns and villages, Greek authorities said.

Hundreds of wildfires have broken out across Greece since May. While summer fires are common in Greece, extraordinarily hot and dry weather linked to climate change have made the blazes more frequent and intense, according to scientists.

More than 560 firefighters backed by volunteers, 17 water-bombing planes and 15 helicopters battled the latest wildfire that broke out Sunday afternoon near Varnavas, 35 kilometres north of the capital.

By Monday morning the fire had spread southward and was burning on several fronts including the village of Grammatiko, the ancient town of Marathon, the seaside municipality of Nea Makri, and Mount Penteli just north of Athens.

Strong winds were constantly changing the flames' direction.

Local media reported two firefighters were slightly injured, while several civilians were treated in hospitals for smoke inhalation. 

"Despite a hard battle and the superhuman efforts overnight, the fire spread very fast and has reached Mount Penteli," a fire brigade official said.

Firefighting aircraft paused operations overnight and resumed efforts early on Monday. Police assisted with evacuations and some residents spent the night in shelters.

At least three hospitals had been evacuated in Penteli.

Dark smoke over Athens

The blaze, with flames as high as 25 metres, spread "like lightning" due to gale-force winds, fire brigade spokesperson Vassilis Vathrakogiannis said on Sunday.

Thick clouds of smoke darkened the sky over Athens on Sunday evening. Hours later, the flames were burning near the residential suburb of Dionysos, about 23 kilometres northeast of the city centre, and nearby areas.

Prime Minister Kyriakos Mitsotakis rushed back to Athens from a weekend break on the island of Crete to oversee the response to the blaze, a government official said, with memories still fresh of a 2018 fire that killed 104 people in the seaside town of Mati, near the capital.

After its warmest winter on record and long periods of little or no rainfall, Greece is forecast to record its hottest-ever summer.

Authorities are on high fire alert until Thursday while weather experts have warned of a "very difficult week."

"Half of Greece will be in the red," Climate Crisis and Civil Protection Minister Vassilis Kikilias said on Saturday, citing temperatures of around 40 C amid strong winds.