A man was reportedly seen with a petrol can close to one of the worst gorse fires for years in the Mourne Mountains, police said today.
Another two youths were spotted lighting fires in south Armagh, reports to the Police Service of Northern Ireland (PSNI) added.
Hundreds of acres of land are being destroyed, and homes and livestock threatened by fires which have burned for much of the bank holiday weekend in counties Down, Armagh and Tyrone.
The fire service said that yesterday was one of the busiest days ever.
PSNI assistant chief constable Gary White said: "The thought that individuals may be deliberately causing such destruction is very hard to comprehend.
"Not only are they putting livestock in danger, and destroying the habitat of wildlife, but as seen in the necessary evacuations in Annalong and the Lenamore Road in Omagh, homes and therefore the lives of residents are being threatened."
Reports of a man being seen with a petrol can close to the youth hostel in Rostrevor, Co Down, on Saturday at around 8pm and two youths lighting fires on Slievegullion Mountain, Co Armagh, at around 7pm last night are two of numerous lines of inquiry being investigated by police.
A fire on the Munie road in Ballymena, Co Antrim, around 5pm on Friday evening is believed to be a deliberate ignition. The blaze claimed around 40 acres of gorseland and numerous trees and at one stage a farm house and outbuildings were in danger.
Mr White said: "It might seem fun and a bit of bravado to set a piece of gorse on fire, but with the current weather conditions a small fire can quickly get out of control, change direction and spread rapidly."
Fire service group commander Philip Martin said crews continued to respond today in a highly professional manner and are working long hours in arduous conditions.
"We are extremely proud of their continued efforts in protecting life and property across Northern Ireland," he said.
"While our resources are heavily committed, I can assure the public that additional resources have been made available to support our operational response."