Hundreds of firefighters in Greece struggled Thursday to tame major wildfires burning for a sixth day, leaving 20 dead and prompting growing outrage among stricken residents.

A dangerous blaze raged for a second day on Mount Parnitha near Athens, in the largest forest adjoining the capital, threatening a national park.

Fire department spokesman Yiannis Artopios told state television ERT there was an "explosion of fire" in a forest ravine early Thursday that renewed the threat to inhabited areas.

In the district of Menidi at the foothills of Parnitha, where many have lost homes, there was anger at the perceived failure of the state to protect properties for yet another summer.

Nikos Lazarou, a 32-year-old mechanic, told AFP he was "furious" about fires "breaking out every year."

The same area had also been hit in 2021 by a major wildfire that burned part of a national park.

"The authorities need to take measures," he said.

"The state really needs to stiffen penalties (for arson), this can't go on, the whole country has burned," Nikos Xagoraris, a local deputy mayor, told ERT before breaking down in tears.

The largest fire front was in northern Greece, where a mega blaze that erupted on Saturday near the port city of Alexandroupoli has now formed a unified front of over 15 kilometres (nine miles).

The bodies of 19 people believed to be migrants, two of them children, were found in the area this week.

Officials have warned that as the area is a popular entry point for smugglers from neighbouring Turkey, more casualties are likely to be found among asylum seekers who could not escape the flames.

A third large fire was in Boeotia, north of Athens, where a 1,000-year-old UNESCO-listed Byzantine monastery, Hosios Loukas, narrowly escaped destruction on Wednesday.

The greater Athens area, alongside Boeotia and the island of Evia were Greek regions most at risk of new fire outbreaks Thursday, the civil protection ministry said.

The hot and dry conditions that increase the fire risk will persist until Friday, according to meteorologists.

Civil Protection Minister Vassilis Kikilias on Wednesday said the country was going through the worst summer since fire-risk maps were introduced in 2009.

"It's an unprecedented situation, this is not a figure of speech," he said.

Greek fire department spokesman Yiannis Artopios on Wednesday said 60 firefighters had been hurt in operations.

The fires have burned over 60,000 hectares (148,000 acres) in northern Greece and another 5,000 hectares west of Athens, according to estimates from the national observatory of forest fires operated by Aristotle University in Thessaloniki.

al-jph/ach