Two firefighting helicopters collided while battling a wildfire in the Psatha area west of Athens, the fire service said on Sunday.
"Search and rescue teams were immediately mobilised to locate and assist the crews," it said.
Greece has been battling wildfires on multiple fronts this weekend, with the prime minister warning of "extremely difficult" days ahead amid winds reaching 100 kilometres an hour.
Nearly 500 firefighters have been working to contain blazes near the coastal village of Porto Germeno, northwest of Athens, while separate fires have also broken out in the northern Peloponnese and on the island of Cephalonia.
Three firefighters have already died in the line of duty this week, two in Crete and one in the Peloponnese, as the fire service faces what the civil protection minister described as being "pushed to its limits."
