Two men aboard one of two firefighting helicopters were killed on Sunday after the aircraft collided during an operation against a large wildfire northwest of Athens, the authorities said, in the latest deadly setback as Greece struggles to contain hundreds of fires.

The helicopters were fighting a blaze in the Attica and Boeotia regions and carrying two Greek nationals and two foreign nationals, the Hellenic Fire Service said. The men aboard the second helicopter survived, it added.

Prime Minister Kyriakos Mitsotakis of Greece said in a social media post on Sunday that the two men who had died were a Greek coordinator and a Danish pilot.

The cause of the collision was not immediately known.

More than 200 fires have broken out across Greece in the past week, the fire service said. As of Sunday, more than 27,000 acres had burned in Attica and Boeotia, according to the National Observatory of Greece.

“The fire risk remains very high. We are going through perhaps the most difficult days of the summer,” Vassilis Vathrakogiannis, a fire service spokesman, said at a briefing on Sunday.

He said ground crews and aircraft were working under “extremely adverse weather conditions,” including strong winds that had initially prevented firefighting aircraft from operating.

Three firefighters were killed last week while battling wildfires on the Greek island of Crete and in the Peloponnese region, according to the fire service. It did not provide details about the circumstances of those deaths.

Greece is among several European countries that have battled major summer wildfires fueled by extreme heat and dry vegetation. The fires have forced hundreds of thousands of people to evacuate.

Wildfires in France and Spain have each become the largest recorded in those countries since the European Union began collecting data. By the end of July, about 250,000 acres had burned in France this year.

In France, Prime Minister Sébastien Lecornu said in a social media post on Sunday that residents evacuated from the southwestern Gironde region, where the country’s two largest fires had burned, were returning home.

“The fires are now under control in our country,” he said, while warning people to remain cautious in the southeastern Var region, where fires continued to flare.

In Spain, Prime Minister Pedro Sánchez said on Thursday that the country’s wildfire emergency was over after fires subsided in the Madrid region. West of Madrid, a fire in Ávila that became Spain’s largest on record was under control with no active flare-ups, Madrid’s Security and Emergency Agency said in a social media post on Sunday.