Islamic State militants are "moving in migrant flows, hiding in plain sight", the UK's defence staff chief has said.
Sir Stuart Peach said he was "worried" about the global reach of IS militants who destroy their identity documents to travel illegally into other countries.
This should lead the UK to think about how it "manages identity", Sir Stuart said.
He said he was concerned about the use of the internet and social media by the group, "popping up all over the world".
It comes as British military personnel arrive in the Middle East to train "moderate" Syrian opposition forces as they attempt to open up a second front around so-called Islamic State's stronghold Raqqa.
Defence Secretary Michael Fallon said the forces will be trained in infantry, medical and explosive hazard awareness skills.
At a London summit with defence ministers from the coalition of countries fighting IS in Iraq and Syria, Sir Michael will reveal that he has agreed to "surge" the number of data recovery experts to exploit material obtained as IS forces are defeated or flee from Mosul in Iraq.
In a lecture to the Royal United Services Institute, Sir Stuart said: "I worry about the global reach of Daesh as an idea - copycat, using the internet - more than the internet - using social media, popping up all over the world.
"The Chinese state has accepted they have a problem.
"This is not now a localised phenomenon, it is now a wider phenomenon.
"And of course we face, as my friend Andrew Parker has said, the director general of MI5, a potential network of combat-experienced terrorists.
"On the other hand they are losing territory rapidly, foreign fighters are being killed and displaced.
"But they are moving in migrant flows, hiding in plain sight.
"One of the obvious deductions, and I think it does bear more scrutiny from the younger and the analytical community, is how we manage identity in a world where people are deliberately trying to destroy their identity documents and/or they move in migrant flows."
Earlier this month, the head of MI6, Alex Younger, said the scale of the terrorism threat to the UK is "unprecedented".