Greece ordered fresh evacuations near Athens on Monday as wildfires spread despite "superhuman" efforts to contain the flames, firefighters said.

At least five more communities were told to flee by civil protection authorities on Monday, after hundreds of people evacuated Sunday from at least eight villages.

"Civil protection forces battled hard throughout the night, but despite superhuman efforts, the fire evolved rapidly," fire brigade spokesman Vassilis Vathrakogiannis said.

"At this moment it has reached Mount Pentelicus and is headed in the direction of Penteli," he added.

The Mediterranean country is exceptionally vulnerable to summer blazes. After the warmest winter on record, Greece also experienced its hottest June and July since reliable data collection began in 1960, and the summer season has seen fires burn daily.

Temperatures are forecast to peak at 39 degrees Celsius (102 degrees Fahrenheit) in Greece on Monday, with wind gusts of up to 50 kilometres (31 miles) per hour.

Two hospitals in Penteli -- one for children and a military facility -- were evacuated at dawn, Vathrakogiannis said.

The brigade has deployed 510 firefighters and 152 vehicles, while 29 aircraft were due to set out at first light, he added.

- 'Everything is burning' -

"Forest fire near you. Follow the instructions of the authorities," said SMS messages sent to people in the Attica region on Monday, indicating in which direction to flee.

Prime Minister Kyriakos Mitsotakis cut short his holiday and returned to Athens Sunday evening to deal with the crisis.

By Sunday afternoon, firefighters had quickly dealt with 33 out of the 40 blazes that had broken out in the past 24 hours.

But the force was battling seven more in the high summer heat, the fire brigade spokesman said Sunday.

Residents of the historic town of Marathon, 40 kilometres (25 miles) east of Athens, were among those ordered to evacuate Sunday.

"Everything is burning," said Giorgos Tsevas, a resident of Polydendri village.

"I have 200 olive trees there but now they are gone," the 48-year-old said Sunday.

Already Saturday, Civil Protection Minister Vassilis Kikilias had warned that half the country was under a high-risk warning for fires due to high temperatures, wind gusts and drought conditions.

"Throughout the night, winds remained strong, creating dangerous situations. Unfortunately their intensity is expected to increase in the coming hours," Vathrakogiannis warned.

Scientists warn that human-induced fossil fuel emissions are worsening the length, frequency and intensity of heatwaves across the world.

The rising temperatures are leading to longer wildfire seasons and increasing the area burnt in the flames, according to the United Nations Intergovernmental Panel on Climate Change.