The director of the Federal Bureau of Investigation has warned that beating the Islamic State on the battlefield will send hundreds of terrorists out of Syria and Iraq to conduct attacks in the West.
Speaking at a cybersecurity conference at Fordham University, James Comey urged the audience not lose focus on what is largely a future threat.
“And that is at some point there is going to be a terrorist diaspora out of Syria, like we’ve never seen before,” he said.
“Not all of Islamic State’s killers are going to die on the battlefield,” he added.
His comments come while US airstrikes are battling to take out Isis strongholds in Aleppo and Raqqa, and while about 40,000 displaced Syrian citizens are starting to return to the embattled city of Fallujah.
Isis has carried out an increasing number of attacks both in the West and in countries nearer its home territories, from France, Germany and Turkey to Lebanon, Afghanistan and Bangladesh.
“Hundreds and hundreds of them [killers], when the coalition succeeds–and I’m confident it will, in crushing Islamic State–through the fingers of that crush are going to come hundreds of really dangerous people,” said Mr Comey.
“And they’re going to flow out primarily towards Western Europe, but we might as well be right next door to Western Europe, given the ease with which people can travel.”
“This is an order of magnitude greater than any diaspora we have seen before.”
Global leaders met in Washington DC last week for a three-day conference to discuss how to defeat Isis, the fist such summit in history.
John Kerry, the US secretary of state, said at the summit that US airstrikes have driven Isis fighters out of nearly half the territory it once occupied in Iraq and out of 20 per cent of its territory in Syria.
US defence secretary Ash Carter added the goal was to eradicate Isis from Afghanistan and Libya next.
Mr Comey compared the Isis network to al-Qaeda, which recruited members from those who had fought the Soviet Union in Afghanistan.
“A lot of terrorists fled out of Afghanistan in the late 80s and early 90s,” he said.
“This is 10 times that or more.”
The war in Syria, which started in 2011 as Middle Eastern uprising began, has displaced millions of Syrians and has triggered the largest refugee crisis since the Second World War.