Hundreds of firefighters in Greece struggled Thursday to gain the upper hand against major wildfires burning unchecked for a sixth day, leaving 20 dead.

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

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.

The largest front was in the north, where a mega fire that erupted on Saturday near the port city of Alexandroupoli has now formed a unified front of over 15 kilometres (9 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 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 of fires since fire-risk maps were introduced in 2009.

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

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.