Swedish firefighters backed by about 340 personnel and firefighting aircraft from several other European Union countries and Norway meanwhile continued to battle a series of forest fires across the country.

The Swedish air force dropped a bomb at noon as part of an unconventional method to tackle the blaze that has engulfed the Tranglset shooting range in the county of Dalarna in central Sweden.

The explosion aimed at depriving the fire of oxygen and the method was to be deployed in a "worst-case scenario," said emergency services spokesman Johan Szymanski.

Undetonated ammunition at the shooting range has hampered efforts to tackle the blaze as firefighters have had to keep a safe distance, and the terrain is very tough.

"Our preliminary assessment is that it has had a very good effect," Szymanski said after the test.

Szymanski said that other fires within 100 metres of the bomb target were extinguished, while fires further away were not fanned.

A jet fighter dropped the bomb from an altitude of 3,000 metres, said Anders Persson of the Swedish air force.

Swedish emergency services, SOS Alarm, on Wednesday afternoon reported 42 fires, up from 25 earlier in the day after the slightly cooler conditions overnight.

The fires broke out earlier this month and have spread quickly due to extremely dry and hot weather conditions. Strong rains have not been seen for weeks, and high temperatures have given Sweden its hottest July for at least 260 years, according to the weather service.

The fires have affected an estimated 250 square kilometres. The Swedish Forest Agency saying that forests worth about 900 million kronor ($138m) have been hit by the blazes, though some of the affected timber can be used for construction purposes.

Australian Associated Press