The Islamic State group is a "global threat" that will take a generation or more to defeat, Washington's envoy for the
US-led coalition fighting the jihadists said Wednesday.
Despite "strategic momentum" against ISIS - or Daesh as he called it - General John Allen conceded, in a keynote speech to the US-Islamic World Forum in Doha, that the fight would continue for several years.
He added that if ISIS were not defeated it could "wreak havoc on the progress of humanity."
"This will be a long campaign," he said.
"Defeating Daesh's ideology will likely take a generation or more. But we can and we must rise to this challenge.
"In an age when we are more interconnected that at any other time in human history, Daesh is a global threat."
In a wide-ranging speech, Allen added that ISIS also poses a new type of threat because of its "depravity."
"As someone who has spent nearly four decades as a United States marine, I have come closer than many to the reality of inhumanity.
"But I have never seen before the kinds of depravity and brutality in this region that ISIL represents and, in fact, that ISIL celebrates," he added, using an alternative acronym for ISIS.
Allen was speaking the day after attending talks in Paris with ministers from around 20 coalition countries.
That meeting followed the fall to ISIS of Ramadi, the capital of Iraq's largest province, Anbar.
That loss has been described as the worst defeat for the coalition since it formed nearly a year ago.
US Secretary of Defense Ashton Carter blamed Iraqi forces, saying there was "an issue with the will of the Iraqis to fight," in comments that angered Baghdad.
Iraq pleaded Tuesday for more global support in the fight against ISIS.
The loss of Ramadi, plus the ancient city of Palmyra in Syria, has led some to question the effectiveness of the coalition in recent weeks.
On the sensitive sectarian issue, Allen said Shiite forces cannot be tasked with holding Sunni areas in the long-term.
"If you liberate a population with a force that is largely or even predominantly Shiite, and that liberating force ultimately for some reason sits on top of that population for some period of time, we may well expect that tensions are going to emerge and then we have the consequent tragedies that we have seen in places," he said.
Despite criticisms, Allen said the coalition had achieved some gains against the extremists and noted that ISIS had been defeated in many places in Iraq and that it has "lost over 25 percent" of the populated territory it once held in the country.
Another area of coalition success, Allen claimed, was its ability to disrupt the group's access to finance.
He said the coalition had gained valuable intelligence on the organization's financial enterprises.
These included extortion, looting, kidnapping for ransom, and human trafficking, said Allen.