The names of the four crew members have not been publicly released.

John McDermott, the founder and president of McDermott Aviation, said his staff’s “thoughts and deepest condolences are with the families, friends and colleagues of those affected by this tragic incident.”

“McDermott Aviation is working closely with the relevant authorities and will continue to assist as they undertake their investigation into the incident,” he added.

Erickson Air-Crane and Chinook helicopters, as well as Canadair water-bombing aircraft are continuing to carry out firefighting efforts.

Early this week, France and Spain battled huge fires – two of the main ones were close to Madrid and Bordeaux respectively. Those have now been contained, though they are still burning. Some 300,000 people who were evacuated have started to return to their homes.

In Greece, media reports describe “a titanic battle” along a swathe of territory that includes Porto Germeno, Psatha, Fokida, Megara and Kefalonia, where more than 500 firefighters have been struggling to bring the situation under control over the past 72 hours.

Most wildfires are sparked by people, whether intentionally or accidentally, but climate change is making the conditions for them more common.

Last winter was mild and wet, encouraging vegetation growth, which has then dried out in successive hot spells, generating fuel for wildfires.