Record temperatures and prolonged drought have fueled wildfires across Europe, with Greece, France, Spain, and Hungary among the worst affected.
Two firefighting helicopters collided head-on while working to control a blaze in the Psatha area of Attica, northwest of Athens, killing two crew members.
The fire service said a Greek and a Danish citizen on one helicopter died at the scene, while a Greek and a British citizen on the other helicopter survived.
An investigation has been launched to determine how the collision occurred.
The fire, which started in Boeotia and spread to Attica, destroyed more than 100 homes.
Preliminary findings suggest the blaze began with a spark on a private power line connecting a wind plant to the main grid, and two Greek citizens were arrested on suspicion of negligence.
As the fire advanced toward Megara, a city of about 30,000 people, authorities issued alerts and began evacuating residents.
New fires on the tourist island of Kefalonia prompted the evacuation of several areas.