Eighteen bodies have been found in a forested area of northern Greece hit by wildfires for the past four days, the Greek fire service says.
Initial reports suggest those who died may have been migrants. A coroner and investigation team are heading to the scene near the Dadia forest, reports BBC.
The Evros region of north-eastern Greece, not far from the Turkish border, has been ravaged by fires.
A hospital in the city of Alexandroupolis had to be evacuated.
Newborn babies and intensive care patients were among those moved to a ferry at the port.
Fires are raging across several fronts in Greece, whipped up by high winds and temperatures which climbed above 40C in several places on Tuesday.
Flames are thought to have spread rapidly since Monday in the large wooded Dadia national park to the north of Alexandroupolis. Emergency services sent mobile text messages to the surrounding areas asking people to leave.
Before the latest grim discovery, an initial death believed to have been of a migrant was reported in the area.
Eighteen more bodies were found on Tuesday near a hut outside the village of Avantas, reports said, when the fire brigade inspected the charred remains of a building.
Fire service spokesman Yiannis Artopios said the possibility that the victims had entered Greece illegally was being investigated, given that there had been no reports of missing residents.
Unconfirmed reports said the bodies were discovered in two groups and there were fears that the number of casualties could increase. The fire service said investigations were continuing throughout the area where the fire had spread.
The Evros region has become one of the most popular routes for Syrian and Asian migrants crossing the River Evros from Turkey into the European Union. The Dadia forest is also known to be a route favoured by migrants.
Yiannis Artopios stressed that emergency messages had been sent to all mobile phones in the area, including foreign networks.
Migrant support group Alarm Phone said it had been in contact with more migrants who needed rescuing from the fires. One group of nine people had already crossed the border while another 250 people were stranded on two small islands in the River Evros, it said.
In the past three days 380,000 acres of land has burned in the Evros region alone, according to the National Observatory of Athens' Meteo unit.
Firefighters are having to respond to major outbreaks in other parts of Greece too. The fire brigade has urged tens of thousands of people to leave parts of the north-west Athens suburb of Ano Liosia.
A few kilometres to the north, 50 nuns were reported trapped when a fire broke out near a historic monastery on the slopes of Mt Parnitha.
Several villages have also been evacuated on the island of Evia and in Boeotia in central Greece.
A fiery, red glow was visible on the fringe of Alexandroupolis in the early hours of Tuesday and satellite images showed several regions of Greece covered in thick smoke.
During the night residents in eight nearby villages were told to leave their homes and head for safety in the city. Later on Tuesday a stream of cars could be seen heading there as vegetation along the coast burned.
Flames were seen entering the grounds of the university hospital while the operation was taking place to evacuate the site on the north-east fringe of Alexandroupolis. Greek officials ordered a fleet of ambulances and buses to take some 115 patients away.
While some of the patients were moved to other hospitals in the city, as many as 90 were taken to a ferry, the Adamantios Korais, which has been requisitioned to look after intensive care patients and new-born babies.
West of Athens, several warehouses became engulfed in flames in an industrial area in Aspropyrgos and close to the Attica Highway the sky darkened with acrid smoke.
Two Albanian workers told the BBC that if helicopters had arrived in time they would have been able to put the fire out.
Around midday on Tuesday a second large fire broke out on the opposite side of the highway in the village of Fyli. Half an hour later residents received a mobile phone message from the 112 emergency number to evacuate the area.
Meanwhile, France endured its hottest ever day on Monday after the mid-August holiday, according to weather service Météo-France.
Temperatures on Monday soared as high as 42.4C in the Drôme area of south-eastern France but the record refers to Monday's daily average temperature of 26.63C, recorded in 30 weather stations across France.
In Switzerland, the high temperatures have pushed the "zero-degree isotherm" - the height where temperatures fall below freezing point - to a record altitude. MeteoSwiss said the limit had now increased to 5,298m (17,381ft).
The point is shifting steadily higher, mainly because of global warming induced by humans, the Swiss met office says.
The increased height of the zero-degree isotherm has been accelerating since the 1970s, especially in spring and summer, it says.