"Unless you're prepared to fight these people on the ground, you may contain them but you won't defeat them," the British Middle East peace envoy Tony Blair said speaking to the BBC in an interview.
Although he admitted that there was "no appetite for ground engagement in the West," he said bombing alone would "not suffice" and at some point "someone's boots on the ground" would be a necessary tactic.
"I'm not saying we in the West need to do this; it would be better if it were done by those people closer to the ground who have got the most immediate and direct interests in fighting them," he added.
"But I don't think we can in all circumstances rule it out and after all we do have the force capability to do this," Blair told the BBC.
In a 6,500-word essay published on his Faith Foundation website, Blair said that there was no way to defeat the ISIL without "a willingness to take casualties in carrying the fight through to the end."
"If it was, we could probably root it out, kill or imprison its leaders, deter its followers and close the doors to new recruits," his essay added.
The UK is part of the recently formed US-led coalition of nearly 40 countries, including 10 Arab states, to stand against ISIL.