The US and its European allies must work together better to stop radicalised Westerners who travel to terrorist training camps and return home to wage attacks, a report said.
The warning came as Western officials investigate an active terrorist plot they said was aimed at carrying out Mumbai-style shooting attacks in Britain, France and Germany.
The new report said radicalised Westerners who easily travel around the world represent a growing terrorism threat. Though they are still fairly few in number, these extremists help inspire and unite others.
It was released by the George Washington University's Homeland Security Policy Institute and the Swedish National Defence College's Centre for Asymmetric Threat Studies,
"Thwarting terrorist travel is of the highest priority," said Charles Allen, a former top US intelligence and homeland security official. "More progress is needed if we are to reach a level of collaboration that gives us the confidence needed to track extremists, who with the proper credentials and a clean record can travel globally."
In the study's foreword, Allen and European Union Counter-terrorism Coordinator Gilles de Kerchove said the report's findings underscored the need for the US and its European allies to improve their information and intelligence sharing, including passenger data.
The report lays out the growing and complex threat from what the authors call Western foreign fighters - Westerners who leave home to train or fight jihad, or holy war, and are often sent back to their countries armed with terror expertise and tasked with launching domestic attacks.
Because these foreign fighters often have different reasons for turning to terrorism and come from a variety of backgrounds, ethnicities and social circumstances, they are hard to spot and track. The report says recent terror incidents in the US - including the foiled New York City subway bombing authorities have blamed on Afghan Najibullah Zazi - help to inspire others.
Western officials must do more to counter that propaganda by highlighting the harsher realities of training camps and giving greater visibility to fighters who have turned away from jihad.
The terror plot unveiled by European officials this week was still in its early stages and not considered serious enough to raise the terror threat level, authorities said. Eight Germans and two British brothers were said to be at the heart of the al Qaida-linked terror plot against European cities.