"The helicopter was attacking the fire at Cabril when it crashed after striking power lines," said Rui Esteves, commander of the ANPC protection agency today.
"It then caught fire, which caused the death of the pilot, the only person onboard," he said.
The pilot, a Portuguese national with long flying experience, had been a member of the fire-fighting force since 2013, he added.
The Ecureuil helicopter, belonging to a private Portuguese aviation firm called Everjets, had been dropping water as the flames headed toward Cabril, a village in the northern Castro Daire municipality.
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This summer has seen a record number of fires in Portugal, and Interior Minister Constanca Urbano de Sousa has blamed arsonists and human negligence for most of them.
Blazes in mid-June near Pedrogao Grande in central Portugal killed 64 people and injured more than 250.
The country has received reinforcements from abroad, including a Canadair water bomber from Morocco, three similar planes from Spain and three Super Puma helicopters from Switzerland.