The two pilots of a firefighting plane died as their aircraft crashed on Tuesday while battling a forest fire on the Greek island of Evia, the country's defense ministry said.

The victims were Greek Air Force officers aged 27 and 34, according to an e-mailed press release. A three-day period of mourning in the armed forces has been announced in their memory.

Greek President Katerina Sakellaropoulou and Prime Minister Kyriakos Mitsotakis expressed condolences to the victims' families and colleagues.

The two officers are the first fatalities of wildfires that have hit Greece this summer.

Separately, state broadcaster ERT reported that the body of a 41-year-old stockbreeder who had been missing since Sunday was found burned in a shack in a hard-to-reach area on Evia.

Firefighters have been battling over 500 wildfires that broke out across the country in the past two weeks, Greek government officials said on Monday.

The fire at Karystos was one of the largest since the weekend, the fire brigade said. The situation remained difficult in parts of the islands of Corfu and Rhodes, where major blazes burning since last week have caused extensive damage in forested zones and forced the evacuation of over 20,000 locals and tourists from several settlements.

Firefighters from 10 countries have already joined their Greek counterparts, and further help is expected to arrive on Wednesday, the fire brigade said.

On Tuesday, Prime Minister Kyriakos Mitsotakis told ministers that the coming days would be difficult, with conditions possibly improving after Thursday.

"All of us are standing guard," he said. "In the face of what the entire planet is facing, especially the Mediterranean which is a climate change hot spot, there is no magical defense mechanism."

The high temperatures in Greece are set to rise through Wednesday to exceed 44 Celsius (111 Fahrenheit) in some areas.

(With input from agencies)