The fire, which started on Wednesday near Dokkedal southeast of the northern city, was extinguished above ground by Thursday morning but is ongoing underground, Ritzau reports.

“The earth here is a bog, so there is a lot of peat under the topsoil. It is burning a long way down,” fire chief Lars Bjørndal of the Nordjyllands Beredskab fire service said.

Underground fire creates a risk of new fires breaking out above ground, Bjørndal said.

“The area out here is surrounded by some quite deep two-metre ditches. They are normally filled with water, but there is not so much at the moment,” he said.

“Our concern is the extent to which the fire can work its way under the ditches and start fires up in new areas,” the fire chief continued.

The fire began at 2:30pm on Wednesday and has burned an area of around 50 hectares above ground, but it is unclear how extensive the underground situation is.

Bjørndal said challenges faced by responders including securing water supplies to the area and finding vehicles that were able to move on the boggy terrain.

“We have some tractors that can break the earth so we can get water down to where it will work. We also have tractors with fertiliser spreaders that can be driven anywhere in the area,” he said.

Around 40 firefighters were working in the area on Thursday morning while up to 70 were present at the scene during the night, Bjørndal said to Ritzau.

No evacuations in the area were necessary, he added.

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