Firefighters struggled on Thursday to douse wildfires that have swept across Sweden after a protracted period of exceptional heat and drought.

Authorities said there were 35 registered wildfires burning on Thursday, affecting more than 20,000 hectares across the heavily forested Nordic country.

In a normal year, Sweden would expect wildfires to affect about 2,000 hectares in total, the Swedish Civil Contingencies Agency (MSB) said on its website.

Several villages in the worst hit municipality of Ljusdal in southern-central Sweden were evacuated overnight. On Thursday Prime Minister Stefan Lofven visited the area, where fires were burning across some 8,500 hectares.

Soldiers, coastguards and civilian groups were helping firefighters to contain the fires.

Two Italian fire fighting planes and Norwegian helicopters were helping with water bombing affected areas. France was also expected to provide two firefighting planes on Thursday after Sweden appealed to the European Union for more outside help.

Forests worth an estimated 600 million Swedish crowns have so far gone up in flames, news agency TT said, citing the Swedish Forest Agency.

Many parts of Sweden have seen their driest weather in the May to mid-July period since records began in the 19th century, the Swedish Meteorological and Hydrological Institute (SMHI) said, and there are few signs of the drought coming to an end.

There have been no reports of deaths or injuries in this year's wildfires. In 2014, one person was killed when the largest wildfire seen in Sweden in 40 years ravaged an area totalling 14,000 hectares in the central Vastmanland region.

Unusually hot weather has gripped Sweden since May, when nearly all regions hit record temperatures for that particular month, in some places by as much as 2 degrees above the average, SMHI said.

In July too, several longstanding records have been broken. The Kvikkjokk-Arrenjarka weather station in Laponia, north of the Arctic Circle, recorded 32.5 degrees, beating a 1945 record of 32 degrees. That was the highest since records began in 1888.