Thugs attempted to relight a major gorse fire in the Mournes after firefighters had battled seven hours to extinguish it.
Thirteen pumping appliances were sent with up to 75 firefighters to fight the fire which damaged up to 50 acres of gorseland in the Newcastle area.
Fire Service commander, Gordon Latimer, said a police helicopter was used to get an aerial view of the blaze.
"It is difficult from the ground to get an appreciation of the scale and the direction of the fire," he said.
"So one of our senior officers travelled in the police helicopter to get an aerial view to help with the operation."
Mr Latimer also said the fire was started maliciously.
"It was a serious fire for a number of reasons.
"The number of resources we had to tie up at this fire meant that service delivery across the province was affected.
"We bring appliances from far and wide, not just from Newcastle, so this dilutes the cover across Northern Ireland."
He said the mountainous terrain and youths relighting fires added to the difficulty of controlling the blaze.
"We were actively fighting the fire for around seven hours. And there were youths seen trying to relight fires in the same area and we informed the police."
Mr Latimer said the fire also posed danger to nearby householders, a caravan site and forestry.
The Mourne fire was just the latest in an number of deliberate gorse fires in Northern Ireland.
Last week around 50 acres of Divis Mountain were damaged.
It was the tenth fire on the Divis and Black Mountains in north Belfast this year.
Mike Dobson, property manager of the National Trust, who looks after the site, said: "I would strongly appeal to those who see the mountain as a great place to create fires to think again."