Tuesday evening’s incident at Lee House, in Peterlee town centre, followed just days after a police warning about the risks of youngsters entering the seven-storey building.
“No individuals were injured and an investigation into the cause of the fire is now underway.”
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A County Durham and Darlington Fire and Rescue Service spokesperson said six crews tackled the fire.
A statement added: “On arrival it was found the fire was on the fifth floor. Crews extinguished the fire using two breathing apparatus, one hose reel and on covering jet. They left the scene at 08.26.”
Peterlee Police’s earlier statement read: “Over the last few months we’ve received regular reports of young people putting themselves in potential danger by getting into the empty Lee House in the town centre.
“Kids are taking a risk by going into a disused building unsupervised, there is evidence of underage drinking, possible drug-taking, damage to windows and evidence of fire-setting.
“Every time this is reported to us, our officers attend and take appropriate action, but we suspect it happens more times than it is reported.
“We are currently in talks with our partners including the council and fire service to secure the site and find a long-term solution to the problem.
“But in the meantime we would urge parents to protect their kids by asking them to stay away.”