Eighteen burned bodies, possibly of migrants, were found on Tuesday in a rural area in northern Greece where wildfires have been burning out of control for a fourth day, the fire brigade said.

The broader Evros region is a popular route for migrants crossing the river from Turkey into Greece.

"Given that there have been no reports of disappearances or missing residents from the surrounding areas, the possibility that these are people who entered the country illegally is being investigated," the fire brigade said. "Searches throughout the entire area where the fire broke out are ongoing."

Another body thought to belong to a migrant was found on Monday.

Earlier on Tuesday, dozens of hospital patients, including newborn babies, were evacuated onto a ferry as hundreds of firefighters struggled to contain the blaze that broke out near Alexandroupolis on Saturday.

It spread quickly, fanned by gale force winds, sending plumes of smoke above the city and turning the night sky red.

Fires also broke out on Tuesday near the capital Athens, where a blaze on the city's outskirts, on the foothills of Mount Parnitha, was threatening homes. An operation was underway to evacuate nuns from Kleiston Monastery, the fire brigade said.

Another blaze burned uncontrolled in the industrial town of Aspropyrgos. More than 120 firefighters and nine aircraft were fighting the blazes near Athens.

By early Tuesday, authorities said 65 patients at the University Hospital of Alexandroupolis had been evacuated as a precaution onto a ferry in the port.

"I've been working for 27 years, I've never seen anything like this," said nurse Nikos Gioktsidis. "Stretchers everywhere, patients here, IV drips there ... it was like a war, like a bomb had exploded."

Fourteen more people were evacuated by a coastguard vessel from a beach near the village of Makri.

Overnight, as flames approached another clinic at the premises of the Alexandroupolis Metropolitan Church Foundation, staff carried a man on a wheelchair to an ambulance, while others were evacuated on stretchers.

A ferry was turned into a makeshift hospital. Elderly patients lay on mattresses on the cafeteria floor, paramedics attended to others on stretchers and a woman held a man resting on a sofa, an IV drip attached to his hand.

The ferry later sailed to the nearby port of Kavala, state broadcaster ERT said.

Communities evacuated
Father Christodoulos Karathanasis, director of the Holy Metropolis of Alexandroupolis, said 200 patients from both facilities had been evacuated in just over four hours.

"Some were able to walk and others were bedridden," he said.

Fire brigade spokesperson Ioanis Artopios said the risk of fire remained high in the coming days.

Fifty-six firefighters arrived in Greece from Romania on Tuesday and Athens was expecting further assistance from the Czech Republic, Croatia, Germany and Sweden with 64 more firefighters, 19 fire engines, seven planes and one helicopter, the fire brigade said.

Several communities in the Evros region have been evacuated as authorities warned the risk of new fires remained high.

"It has reached the entire village," said Alexandros Chrisoulidis, a 19-year-old resident of Avanta village. "Our own house up there, where the fire started, has completely burned down. There is nothing," he said.

A 23-year-old resident who gave his name as Nikos, said: "The situation is tragic. All that is needed right now are prayers and rain."

Summer wildfires in Greece are common but have been made worse in recent years by unusually hot, dry and windy conditions that scientists have linked to climate change.