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."

In pictures: Iraq battles to drive Isis out of Fallujah Show all 12 1 /12 In pictures: Iraq battles to drive Isis out of Fallujah In pictures: Iraq battles to drive Isis out of Fallujah Smoke rises after airstrikes by US-led coalition planes as Iraqi security forces advance against Islamic State extremists in Fallujah, June 15, 2016 AP In pictures: Iraq battles to drive Isis out of Fallujah Iraqi security forces advance during heavy fighting against Isis militants in Fallujah, Iraq, on 14 June AP In pictures: Iraq battles to drive Isis out of Fallujah Shia militia say that moving resources from Fallujah towards the area near Mosul was a 'betrayal' of the battle for the city GETTY In pictures: Iraq battles to drive Isis out of Fallujah Hospital sources said 18 bodies were recovered from the river over the weekend AP In pictures: Iraq battles to drive Isis out of Fallujah Up to 60,000 civilians were feared trapped in Fallujah at the start of the Iraqi operation AP In pictures: Iraq battles to drive Isis out of Fallujah Shia fighters hold an Isis flag in an operation east of Fallujah – the terror group has lost ground in both Syria and Iraq AFP/Getty In pictures: Iraq battles to drive Isis out of Fallujah Shia fighters hold their weapons as they gather near Falluja, Iraq, June 4, 2016. Reuters In pictures: Iraq battles to drive Isis out of Fallujah Pro-government forces bid to take back ground from Isis in Fallujah MOADH AL-DULAIMI/AFP/Getty Images In pictures: Iraq battles to drive Isis out of Fallujah Smoke billows on the horizon as Iraqi military forces prepare for an offensive to retake the city AP In pictures: Iraq battles to drive Isis out of Fallujah A member of the Iraqi security forces fires artillery during clashes with Isis militants near Fallujah, Iraq, 29 May, 2016 Reuters In pictures: Iraq battles to drive Isis out of Fallujah Iraqi government forces fire a rocket near al-Sejar village, north-east of Fallujah, on May 26, 2016, as they take part in a major assault to retake the city from the Islamic State group AFP/Getty In pictures: Iraq battles to drive Isis out of Fallujah Shia fighters and Iraqi security forces advance towards Fallujah Thaier Al-Sudani/Reuters

“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.

In pictures: Isis' Libyan stronghold of Sirte Show all 10 1 /10 In pictures: Isis' Libyan stronghold of Sirte In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte Isis fighters parade through in Sirte in 2015 In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte Islamic State (IS) group jihadists on the outskirts of Libya's western city of Sirte AFP/Getty In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte A photo of a billboard in Sirte, Libya, listing seven rules for women's clothing, saying they must be loose-fitting and undecorated HRW/social media In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte Isis militants process down a street in the coastal city of Sirte in Libya this week; the group has heralded Libya as its ‘strategic gateway’ to attack Europe AFP/Getty Images In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte An Isis lecture on Sharia at the Ouagadougou complex in Sirte, Libya, in 2016. HRW/social media In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte A sign reading "The city of Sirte, under the shadow of Sharia" as smoke rises in the background while forces aligned with Libya's new unity government advance on the eastern and southern outskirts of the Islamic State stronghold of Sirte on 9 June. Reuters In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte Fighters loyal to Libya's GNA prepare to launch attacks against Isis as they continue their resistance on the outskirts of the western city of Sirte Getty In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte Forces loyal to Libya's UN-backed unity government are seen during clashes with jihadists of the Islamic State (IS) on the western outskirts of Sirte on June 2, 2016. AFP/Getty Images In pictures: Isis' Libyan stronghold of Sirte Isis in Sirte Forces loyal to Libya's UN-backed unity government fire during clashes with Isis around 14 miles west of Sirte on June 2, 2016. AFP/Getty Images In pictures: Isis' Libyan stronghold of Sirte Isis in Libya

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.”