The EU's anti-terror chief has told the BBC that hundreds of Europeans are now fighting with rebel forces in Syria against Bashar al-Assad's regime.
Gilles de Kerchove estimated the number in Syria at about 500.
Intelligence agencies are concerned some could join groups linked to al-Qaeda and later return to Europe to launch terrorist attacks.
The UK, Ireland and France are among the EU countries estimated to have the highest numbers of fighters in Syria.
"Not all of them are radical when they leave, but most likely many of them will be radicalised there, will be trained," Mr de Kerchove told the BBC.
"And as we've seen this might lead to a serious threat when they get back."
Across Europe, intelligence agencies have stepped up investigations, says the BBC's Europe correspondent Duncan Crawford.
In Britain and Belgium they have increased efforts to track how people are recruited.
In the Netherlands, officials have raised the terror threat level there to "substantial" - partly over concerns about radicalised citizens returning from Syria.