Two people on a firefighting helicopter — a Greek and a Danish national — died on Sunday after their helicopter crashed after a mid-air brush with a second helicopter.
The crew of the second firefighting aircraft, a Greek and a Briton, survived the incident near the seaside community of Psatha, about 60 kilometres from Athens.
Strong firefighting forces assisted by water tanks and 21 aircraft were deployed near the communities of Kandili and Agia Skepi early on Monday as plumes of thick black smoke and red flames engulfed lush pine forests.
Local authorities with heavy machinery built fire breaks to stop the wildfire from reaching homes, a Greek fire brigade official said.
Europe has been ravaged by wildfires this summer after a period of record-breaking heatwaves and little rain — conditions that scientists say have been exacerbated by a fast-changing climate.
France and Spain have been particularly hard hit although destructive blazes there abated over the weekend, providing some respite.
Sunday's air crash follows a grim week for Greek emergency services. Three firefighters were killed in two separate incidents in Crete and on the Peloponnese on July 29 in a jarring start to a fire season which has, until now, been relatively mild by local standards.
Greek authorities are trying to ascertain the cause of the mid-air crash of the two helicopters, which occurred as they were flying at low altitude, with the rotor blades of one appearing to clip the underside of the other, footage which appeared on social media showed.
"We're fighting an unequal battle with the impact of the climate crisis," Deputy Climate Crisis Minister Kostas Katsafados told public broadcaster ERT, adding that strong winds over the past few days made water bombers unable to draw sea water and dump it in the area, leaving ground forces battling the blazes on their own.
Preliminary investigation suggests the outbreak in the area of Boeotia, northwest of Athens last week, which spread to a lush pine forest near the capital, was caused by sparks from vibrating conductors on a private power line transmitting electricity from wind turbines, the Greek fire brigade said.
Two Greek nationals, an electrical engineer and a contractor, have been arrested and face felony charges of arson.
Faulty power lines have been the leading cause of major wildfires in Greece in recent years, overshadowing arson and negligence.
