Up to 5000 European Union citizens have travelled overseas to fight as jihadists, police agency Europol has revealed.
Speaking to a British parliamentary committee, Europol chief Rob Wainwright said the “large body” of men posed a threat to their country of origin if they returned.
“We’re talking about 3000, 5000 EU nationals,” Mr Wainwright told British lawmakers.
“We’re dealing with a large body of mainly young men who have the potential to come back and have the potential or intent and capability to carry out attacks we have seen in Paris in the last week,” Mr Wainwright said.
He said the returning militants were “the most serious terrorist threat that Europe has faced since 9/11” 2001 attacks.
Mr Wainwright called for greater scrutiny of the use of social media, which he said was used as a recruitment tool by jihadists.
“One of the important evolutions we’re seeing right now in the current terrorist threat is the way the internet is used, clearly much more aggressively, much more imaginatively by the networks,” he added.
The names of about 2500 suspects have been collected by Europol from the security services of various EU countries, Wainwright said.
The European Union’s counter-terrorism chief Gilles de Kerchove estimated in September 2014 that around 3000 European citizens had joined jihadists in Syria and Iraq.
De Kerchove said in an interview with AFP in Brussels on Tuesday that around 30 per cent have returned to their EU countries.
The issue of home grown jihadists has gained prominence in recent days following a string of attacks in Paris by assailants born in France.
One of two brothers responsible for an attack on the French satirical magazine Charlie Hebdo last week, Said Kouachi, studied at a Yemen college founded by a fundamentalist cleric before undertaking weapons training with an al-Qaeda affiliate.
His brother Cherif was part of the so-called Buttes Chaumont network in Paris, which helped send recruits to join the Iraqi branch of al-Qaeda.
– with AAP