European anti-terrorism chief Gilles de Kerchove said he foresees problems when European citizens helping the anti-Assad opposition return home. Most of them, he said, are likely to be radicalized by the experience of the war in Syria.
"Not all of them are radical when they leave, but most likely many of them will be radicalized there, will be trained," de Kerchove told the BBC. "And as we've seen this might lead to a serious threat when they get back."
De Kerchove has been warning of Europe's increasing exposure to terrorism since his appointment in 2007 in response to the 2004 bombing of Spanish commuter train system in Madrid that killed more than 190 people and wounded 1,800 others.
A Spanish government investigation found the attacks were inspired by an al-Qaida inspired terrorist cell.
Nearly a decade later, de Kerchove's comments cited a transformation of the threat to Europe because of a growing European involvement in Syria. While more than 500 European citizens are known to be fighting alongside opposition forces in Syria various EU initiatives are helping to arm the rebels.
The European Union recently eased a Syrian oil embargo to enable rebels controlling oil installations to sell their oil abroad. It's not clear what oil facilities the rebels control and whether the measure is to prepare for future events.
Cross-party EU support for the Syrian rebels has wavered in recent weeks amid reports that the opposition groups include al-Qaida-linked groups that aim to oust Assad, but not to supplant him with a pro-West regime.
Al-Qaida-linked groups were reported active in Syria last year but their activities were played down amid EU enthusiasm for mounting an independent foreign policy initiative in Syria.
Security officials in the United Kingdom have said that up to 100 British Muslims have gone to fight in Syria, the BBC said.
The officials fear the conflict isn't only attracting young men who sympathize with the rebels but also aspiring jihadists who want to train and fight.
The EU citizens who go to Syria for cooperation with opposition groups in the Arab country increase the risk of future terror plots in Europe, analysts say, echoing comments by de Kerchove.
Europol pan-European police force said in an annual report on terrorism trends that Syria was the destination of choice for foreign fighters in 2012.
Europeans returning after fighting and training in Syria posed a major threat to Europe, the report said.
It said the threat had widened to include structured groups and networks, smaller EU-based groups and lone operators.
This week de Kerchove told the BBC Britain, Ireland and France were among EU countries that have the highest numbers of militants in Syria.
EU foreign ministers met in Luxembourg Monday to consider European options in Syria. The decision to ease the oil embargo was the only point to emerge from the largely secret discussions.