The blaze in the the Alexandroupolis and Evros region is mainly concentrated deep in a forest near the border, in an area difficult to access.
The wildfire has been blamed for 20 of the 21 wildfire-related deaths in Greece last week.
Six planes and four helicopters were assisting 475 firefighters on the ground, backed by 100 vehicles, the fire department said.
The wildfire, which broke out on August 19, had scorched more than 80,000 hectares of land by Monday night, according to the European Union’s Copernicus Emergency Management Service.
EU officials have described it as the largest single blaze recorded in the bloc since the European Forest Fire Information System began recording data in 2000.
European Commissioner for Crisis Management Janez Lenarcic said: “We stand in solidarity with Greece as it battles the devastating wildfires. The European Union’s largest aerial firefighting operation underscores our commitment to swift and effective collective action in times of crisis.
“Our thoughts are with the people of Greece, and we will continue working tirelessly to protect lives, property, and the environment. The EU’s unity and cooperation are our strongest assets in overcoming these challenges.”
Responding to the largest #wildfires🔥 ever recorded in the EU, we are conducting a massive #rescEU operation in Greece with:
🔺️11 firefighting planes
More on our #EUCivilProtection operation👇https://t.co/6FPGTYMp5H pic.twitter.com/UPZZn1NXHw
— Janez Lenarčič (@JanezLenarcic) August 29, 2023
Another 260 firefighters and one helicopter were tackling flare-ups of another major blaze burning for days in a forest on the southern slopes of Mount Parnitha, on the fringes of the Greek capital.
Authorities are investigating the causes of the fires, which over the past week have destroyed vast tracts of forest, scorched homes and triggered the evacuation of thousands of people.
“In total, we have mobilised 12 aircraft stationed in seven member states. This is the biggest rescue aerial firefighting operation to date” since the emergency response mechanism was launched in 2019, the EU Civil Protection and Humanitarian Aid authority said on Monday on X, formerly known as Twitter.
In Athens, Prime Minister Kyriakos Mitsotakis was holding a government meeting on how to rehabilitate the burnt forests in Evros and Parnitha.
With firefighting forces stretched to the limit, Greece has called for help from other European countries, receiving aircraft and hundreds of firefighters from across the continent.
France and Spain were sending additional aircraft to join those already sent from Germany, Sweden, Croatia, Cyprus and the Czech Republic under the European Union’s emergency response mechanism.
More than 350 firefighters have also been sent to Greece from Romania, France, the Czech Republic, Bulgaria, Albania, Slovakia and Serbia.
Across the country, firefighters were battling 74 wildfires on Monday, with 27 of them having broken out in the 24 hours between Sunday evening and Monday evening, the fire department said.
Arson is suspected in some of the blazes, with several people arrested.
Greece imposes wildfire prevention regulations, typically from the start of May to the end of October, limiting activities such as the burning of dried vegetation and the use of outdoor barbecues.
By Friday, fire department officials had arrested 163 people on fire-related charges since the start of the fire prevention season, government spokesman Pavlos Marinakis said, including 118 for negligence and 24 for deliberate arson. The police had made a further 18 arrests, he said.
The broader Evros region is a popular route for migrants crossing the river by the same name from Turkey into Greece.