HELSINKI • Finland is battling its worst forest fire in half a century with some 300ha ravaged in five days in a north-western valley during an exceptionally hot summer.

The fire began on Monday about 25km south of the small port town of Kalajoki in the Gulf of Bothnia.

"When it started spreading as a treetop fire, we measured its spread as fast as 20kmh," firefighting operations chief Jarmo Haapanen told Reuters.

While some rain had fallen, it will take at least another week to put out the blaze as the smouldering forest base continues to pose a danger, he said, adding: "There is still risk it might spread if the drought continues and the wind starts blowing."

About 250 people, including army soldiers, and four helicopters have been mobilised to douse the flames.

There have been no casualties so far as the area, located about 500km north of the capital Helsinki, is sparsely populated.

Experts say this was the biggest fire in Finland since 1971 when a forest fire consumed 1,600ha.

Forest fires are rare in Nordic countries and are quickly brought under control.

An exceptionally long heatwave saw Finland experiencing record temperatures this summer, with June averaging 20 deg C, some four to five degrees above historical averages.

Scientists at Finland's Meteorological Institute and the Natural Resources Institute concluded in a research paper that rising temperatures will make conditions more favourable for fires in Nordic forests.

"In short, it means that transpiration intensifies and the terrain dries faster," meteorologist and research scientist Ilari Lehtonen of the Meteorological Institute told Reuters.

AGENCE FRANCE-PRESSE