The fire service says young people were seen fleeing the scene after starting a fire in York last night (Thursday).
An Acomb crew were called to reports of a small fire in the open at 9.11pm.
A North Yorkshire Fire & Rescue Service spokesperson said: “On arrival crews found this to be a small wooden den on fire, started deliberately by youths seen running away.
“Crew extinguished using buckets of water and knapsack sprayers.”
Police have been informed.
