Gilles de Kerchove estimates there are currently around 500 European fighters in Syria, the BBC reported on Wednesday.
Those estimated to have the highest numbers of fighters in Syria include the UK, Ireland and France.
De Kerchove told the BBC that while not all of them are "radical" when they arrive in Syria "most likely many of them will be radicalized there".
"And as we've seen this might lead to a serious threat when they get back," he added.
Intelligence agencies are reportedly concerned that some of these fighters could develop links with al-Qaeda  and later return to Europe to launch terrorist attacks.
Britain and Belgium have increased efforts to track how people are recruited