Western security officials are worried about a wave of "fast turnaround" volunteers who travel to Pakistan and obtain training from militant groups so quickly they escape detection before returning to their home countries to launch attacks.
Analysts say the unprecedented speed with which new militants are being accepted for training by groups such as al Qaeda poses problems for intelligence services as such individuals are likely to stay "below the radar".
Security forces say fears have been reinforced by a recent episode when British volunteers arrived in the southern Pakistani city of Karachi, found their way to a religious school with a reputation as a gateway to militant groups and were within days participating in a training course run by al Qaeda or a linked extremist organisation in the tribal zone along the Afghan border.
After only a short stay in Pakistan, the volunteers had returned to Britain.
Previously volunteers would have had to travel with reliable references from individuals known and trusted by extremist groups in Pakistan and would spend weeks "in quarantine" before being accepted.
Frequently they would be tested in combat or in other ways to ensure they were not spies.
Richard Barrett, head of the expert committee established by the United Nations Security Council to oversee sanctions against the Taliban and al Qaeda, said: "People are going in for a shorter time and are much harder to spot.