Thousands of people have fled their homes on the Greek island of Evia as wildfires burned uncontrolled for a sixth day on Sunday, and ferries were on standby for more evacuations after taking many to safety by sea.

The blaze on Evia, Greece's second-biggest island, quickly burgeoned into several fronts, ripping through thousands of hectares (acres) of pristine forest across its northern part, and forcing the evacuation of dozens of villages.

The flames engulfed homes in five villages but the full extent of the damage was not immediately known.

"(It's) like a horror movie," said a 38-year-old pregnant evacuee who gave her name as Mina, after she boarded a rescue ferry at the town of Pefki, where falling ash covered the port.

"But now this is not the movie, this is real life, this is the horror that we have lived with for the last week," she said.

Wildfires have broken out in many parts of the country during a week-long heatwave, Greece's worst in three decades, with searing temperatures and hot winds creating tinder-box conditions. Across the country, forest land has burned and dozens of homes and businesses have been destroyed.

Since Tuesday, the coastguard has evacuated more than 2,000 people, including many elderly residents, from different parts of Evia, which is linked to the mainland by bridge, in dramatic sea rescues as the night sky turned an apocalyptic red.

Others fled their villages on foot overnight, walking along roads dotted with trees in flames.

"A house is burning over here," one woman told emergency crews on the ground in the settlement of Vasilika, pointing to a searing fire in the distance.

"Everywhere, everywhere, everywhere, everywhere," one of the firefighters replied.

The governor for central Greece, Fanis Spanos, said the situation in the north of the island had been "very difficult" for nearly a week.

"The fronts are huge, the area of burned land is huge," he told Skai TV. More than 2,500 people have been accommodated in hotels and other shelters, he said.

Greece has deployed the army to help battle the fires and several countries including France, Egypt, Switzerland and Spain have also sent help including firefighting aircraft.

More than 570 firefighters are battling the blaze in Evia, where two active fronts were burning in the north and south of the island.

In the village of Psaropouli, evacuated residents said they were angry.

"I lost my home ... nothing will be the same the next day," one woman who gave her name as Vasilikia said.

"It's a disaster. It's huge. Our villages are destroyed, there is nothing left from our homes, our properties, nothing, nothing," she said.

Greece's deputy civil protection minister, Nikos Hardalias, said emergency crews were undertaking "superhuman efforts" against multiple fronts.

"The night ahead will be difficult," he said during an emergency briefing late on Sunday. Earlier, he said water-bombing aircraft in the region faced several hurdles including low visibility caused by the thick plumes of smoke rising over the mountains and turbulence.

A fire in the foothills of Mount Parnitha that swept through suburbs north of Athens had been contained but weather conditions meant there was still a high threat it could flare up again.