ATHENS, Greece (AP) — A massive wildfire was burning unabated in northeastern Greece for the 11th day Tuesday despite the efforts of hundreds of firefighters and a fleet of water-dropping aircraft from Greece and several of its European Union partners.

After burning across vast tracts of land, the blaze in the Alexandroupolis and Evros region was mainly concentrated deep in a forest near the border with Turkey, in an area difficult to access.

The wildfire, which was blamed for 20 of the 21 wildfire-related deaths in Greece last week, is the biggest in the EU since the European Forest Fire Information System started keeping records in 2000.

Six planes and four helicopters were assisting 475 firefighters on the ground, backed by 100 vehicles, the fire department said. Another 260 firefighters and one helicopter were tackling flare-ups of another major fire 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 large tracts of forest, scorched homes and triggered the evacuation of thousands of people.

In Athens, Prime Minister Kyriakos Mitsotakis chaired a government meeting on how to rehabilitate the burnt forests in Evros and Parnitha. It decided to identify all burned areas for reforestation, take action to prevent flooding later in the year and provide access to food and water for the area’s surviving wildlife.

With firefighting forces stretched to the limit, Greece has called for help from other European countries, receiving 12 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 EU’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.

The top EU official for crisis management, Janez Lenarcic, said Tuesday it was the bloc’s largest aerial firefighting operation and “underscores our commitment to swift and effective collective action in times of crisis.”

The Alexandroupolis and Evros wildfire, which broke out on August 19, has scorched around 81,000 hectares (200,000 acres) of land, according to the European Union’s Copernicus Emergency Management Service. Copernicus is the EU space program’s Earth observation component and uses satellite imagery to provide mapping data.

Across Greece, firefighters were battling 87 wildfires on Tuesday, with 44 of them having broken out in the 24 hours between Monday evening and Tuesday evening, the fire department said.

Arson has been 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.

As of Friday, fire department officials had arrested 163 people on fire-related charges since the start of the fire prevention season, including 118 for negligence and 24 for deliberate arson, government spokesperson Pavlos Marinakis said. Police made an additional 18 arrests, he said.