Police said they were alerted about reports of the fire just after 8pm.
Inquiries were said to be at an early stage and officers have requested members of the public not to speculate about what happened.
Emergency services attended and the fire was extinguished a short time later.
A police statement said: “Subsequently, last night we issued an appeal for missing boy Layton Carr – it was believed he was in the area at the time.
“Sadly, following searches carried out by Northumbria Police and partners at Tyne and Wear Fire and Rescue Service, a body believed to be that of 14-year-old Layton Carr was located deceased inside the building.
“His next of kin have been informed and are currently being supported by specialist officers.”
‘Extremely tragic’
Northumbria Police Detective Chief Inspector Louise Jenkins said: “This is an extremely tragic incident where a boy has sadly lost his life.
“Our specialist officers will continue to support them in whatever way they can. We ask that their privacy be respected at this time.”
She added: “A cordon remains in place while we carry out inquiries to establish the full circumstances surrounding the incident.
“I’d like to thank our officers and partners at Tyne and Wear Fire and Rescue Service for their efforts through the night.”
