The disturbing revelation was made by Foyle District Commander, Willie Lynch, who told the Journal: "The biggest part of our workload - I would estimate 90% - is fires started deliberately."

Speaking after his crews answered two call-outs to extinguish fires started by arsonists in Strathfoyle and Donemana on Saturday evening, Mr. Lynch said the stark possibility of the Fire Service being overstretched was "very real and always at the back of your head."

He said: "While people might say it is our job to fight fires we can't be in two places at the same time."

His comments follow the dramatic rescue of three people from two upstairs flats in Strathfoyle. The Fire Service control room dealt with a number of 999 calls, some of which stated that as many as seven people could be trapped in the building. The drama started when a number of sparklers were taped together and shoved through a letterbox.

Mr Lynch said: "The fire itself was quite small but there was a lot of smoke."

After rescuing those trapped inside, one person was taken to hospital for the effects of smoke inhalation.

Three fire appliances and 20 personnel attended the scene. While there, the Fire Service received a call to a blaze which "totally destroyed" a Donemana farm building and its contents.

"Two appliances were despatched to the Duncastle Road incident from Strabane but part of that road is serviced by Crescent Link Station. If it had been that part, then we could have been overstretched by fires which were totally avoidable," said Commander Lynch.

A spokesperson for the PSNI said of the firework attack: "Officers would like to remind parents to remind children of the dangers of this type of behaviour and that any type of firework should be used safely and under supervision."

The spokesperson then appealed for information on the Duncastle Road fire. "Anyone who saw suspicious activity in the area between 7pm and 7.45pm is asked to contact Strabane station on 0845 600 8000 or the 'Crimestoppers' charity on 0800 555 111."