A fire is burning one of the world's tallest trees

US authorities are trying to extinguish a fire that is burning in one of the world's tallest trees in Oregon.

The Doerner Fir, a coastal Douglas Fir tree over 99 metres tall and estimated to be over 450 years old, has been burning since Saturday in Coos County in Oregon's Coast Range.

An infrared drone flight on Tuesday showed no active flames or smoke at the top of the tree, but it detected heat within a cavity in the tree trunk some 85 metres high, federal Bureau of Land Management spokesperson Megan Harper said.

Figuring out how to approach the tree from the side to douse the cavity with water has been a challenge, Harper said.

Various options have been discussed, including building scaffolding or climbing adjacent trees for better positioning, or letting it smoulder and monitoring to see if it reignites.

Crews remained onsite Tuesday and a helicopter was on standby in case of needed water drops, she added.

The Coos Forest Protective Association said on Monday that helicopter bucket drops had reduced fire activity near the top of the tree. It added that sprinklers were placed at the base of the tree, where containment lines were also constructed to prevent further fire spread.

The fire may impact the tree's standing in global height rankings, Harper said.

"We've lost about 50 feet (15 metres) of it, just from fire and pieces falling out," she said, noting that the 15 metres were lost through the top burning.

"So I don't know where it'll stand after this, but it's still a magnificent tree."

The cause of the fire has not been determined and is under investigation. It is the only tree on fire in the immediate vicinity, and investigators are looking at lightning data to see if there were any recent strikes in the area, Harper said.

For now, the tree is not at risk of fully burning down, according to Harper.

"Right now, that is not a danger," she said.

"The tree is so big, it's got so much mass that it would take a while for it to burn all the way through the tree."