BRUSSELS: The departure of hundreds of young Europeans to fight in Syria poses “a serious threat” to Europe's security, the EU's anti-terror chief Gilles de Kerchove said Thursday.

EU citizens were heading “in the hundreds” to Syria which was drawing foreign fighters “in the thousands if we aggregate with those leaving from the Balkans and North Africa,” he said.

Speaking to the European Parliament's home affairs committee, the EU intelligence expert said “the core of our threat remains related to Al-Qaeda”despite the group's “very much degraded” core in recent years.

Along with the proliferation of Al-Qaeda affiliates touting regional agendas -- such as Al-Qaeda in Syria or Al-Qaeda in the Maghreb, or AQIM -- a “worrying development is the attraction of Syria” to youngsters from the 27-nation bloc, he said.

“If they are not killed there, they will pose a serious threat to our security,” de Kerchove said.

“How can we stem the flow and face the return?” he asked, saying that it was “a difficult phenomenon” to face up to as some of the volunteers were romantics while others were “radicalised” militants sent to Syria through networks.

“We need to see what we can do to prevent these individuals departing and (then to) cope with their return,” he said.

Police, judicial experts and the intelligence community were examining the problem to come up with solutions. “We are in a brainstorming stage.”Some countries might want to look at adopting legislation to outlaw fighting on foreign fronts.

Another key would be to organise to gather together intelligence and travel data, Kerchove said.

He regretted that the European Parliament just this week rejected a Brussels plan to allow European air passenger data to be used to fight organised crime and terrorism.

“I'm convinced a new PNR (Passenger Name Record data) would provide information to prevent this flow of fighters,” he said. It was also vital to improve coordination with countries such as Turkey through which the volunteers travel.

Belgium last week went on the offensive against radical Islamist recruitment networks, staging dozens of early morning raids and several arrests of individuals suspected of sending foreign fighters to Syria.

A report released early this month by King's College London said up to 600 people from 14 countries, including Austria, Britain, Germany, Spain and Sweden had taken part in the Syria conflict since it began in March 2011.

The largest contingent was from Britain but based on population, the figures for Belgium, Ireland and the Netherlands were the most significant, with around 200 between them.