MADRID (AP) — A major wildfire in northeastern Spain that began in a pile of chicken dung, raged out of control for a third straight day Friday.

More than 600 firefighters and six water-dropping aircraft were battling the blaze in the Catalonia region as Spain is forecast to endure the peak of a recent heatwave, with temperatures expected to exceed 40 degrees Celsius (104 Fahrenheit).

Officials say cooler overnight temperatures allowed crews to slow the fire's advance, but authorities say it is the region's worst fire in two decades and 20,000 hectares (50,000 acres) of hilly terrain are at risk.

Fire investigators believe the blaze started with the spontaneous combustion of a pile of chicken manure on a livestock farm.

Chief Inspector Josep Antoni Mur told a news conference Thursday that the pile grew hotter as it fermented, and then flames were blown into the surrounding vegetation, private news agency Europa Press reported.

Miquel Buch, the regional interior minister, asked people to exercise extreme caution in forested areas.

He says fire services are at full stretch "and we can't allow another wildfire to break out."

Authorities were caring Friday for more than 50 people evacuated from the previous day. They included locals as well as people from Britain, Belgium, Germany, Brazil and the United States, according to Europa Press.

Numerous local roads were closed.