Owners of disused buildings in north Wales are being warned to make their premises secure after two suspicious fires in the past couple of days.
An investigation is under way after an arson attack damaged a derelict Denbighshire seafront hotel on Friday.
It followed an incident at a former hospital in Flintshire the previous day.
The fires could have developed unnoticed and put lives "at serious risk", said North Wales fire service.
Owners of empty buildings are being urged to be vigilant.
Fire crews took more than an hour to control the blaze at the Grade II-listed former Grange Hotel in Rhyl early on Friday.
An annexe and part of the ground floor of the main building were destroyed in the incident.
The disused 29-bedroom hotel, built in the 1900s, was damaged by fire in 2008.
Early on Thursday, firefighters tackled a blaze at the former Lluesty Hospital in Holywell. The fire was confined to the first floor.
Fire service arson reduction manager Kevin Jones said: "It appears that both fires were started deliberately.
"Fortunately no one was injured but these incidents are worrying as they present a serious possibility that fire could have developed unnoticed putting lives at serious risk.
"As it was, these reckless acts tied up life saving resources putting the lives of other people, including firefighters, at risk."
He said deliberate fires were a serious form of anti-social behaviour and would not be tolerated.
"North Wales Fire and Rescue Service is working closely with North Wales Police and other partner agencies to try and eradicate this problem," he said.
He said prosecutions would be pursued and anyone with information was asked to report it to North Wales Police on 101 or Crimestoppers on 0800 555 111.