The flow of European citizens joining rebel forces in Syria has given fresh impetus to efforts to introduce legislation making it a criminal offence to fight or train with jihadist groups overseas, the EU’s counter-terrorism chief has said.

In an interview with The Irish Times, Gilles de Kerchove says several EU member states have become increasingly anxious about the numbers travelling from Europe to fight with rebels battling Syrian president Bashar al-Assad.

“We don’t have a precise figure but my own estimate is between 600 and 800,” he says.

According to research by The Irish Times, based on interviews with fighters on the ground in Syria, up to 20 people from Ireland have joined the ranks of Syrian rebels since the uprising began in 2011. De Kerchove concurs with this. Three have been killed there.

Most of those who have travelled from Ireland have done so under the auspices of Liwa al- Umma, a brigade initially founded by a Libyan-Irish man, Mehdi al-Harati, and now aligned with the Free Syrian Army.

They have not joined more hardline groups like Jabhat al- Nusra, which the UN sanctioned last month because of its links to al-Qaeda.

Harati and a number of others from Ireland involved in Liwa al-Umma have not been back to Syria since last autumn when Turkish authorities informed them they were no longer allowed to enter Turkey.

De Kerchove says that while he estimated the “bulk” of European fighters in Syria are “very much inspired by the al-Qaeda narrative”, it is important to make the distinction between more radical types and what he calls “idealists who are going there like the French writer Andre Malraux went to Spain” during its civil war in the 1930s.

“The estimated percentage differs from one member state to another,” he says. “There is a problem . . . in that the more we insist on these phenomenon, the more we seem to give substance to statements by [Assad], the Iranians and the Russians that this is not a question of a quest for democracy by free Syrian people, but a plot by al- Qaeda to impose a caliphate.

“We support the opposition but at the same time we don’t want to let it develop into such a project.”

There are concerns in some European countries that EU citizens who take up arms in Syria could return radicalised and turn their focus on domestic targets. “It makes sense that they are prepared to handle the returnees in the best way possible . . . If you have been fighting for more than a year in Syria, returning to a peaceful, civilian life is not easy,” he adds.

“One of the concerns I have heard is that even if they go to Syria with this idealistic view, being trained, acquiring battlefield experience and being exposed to the alphabet soup of extremist groups, there will have some impact on their beliefs.

“The experience there is very harsh, this is a very ugly war with the Syrian army killing in a very indiscriminate manner and possibly using chemical weapons, so it might not turn them into jihadists . . . but it may have an impact.”

France has draft legislation which would seek to criminalise committing acts of terrorism overseas or going abroad to receive training with jihadist outfits. The UN decision to sanction Jabhat al-Nusra means that French authorities are now already able to prosecute citizens who return from Syria if they fought for the group.

“Providing the evidence is a challenge in itself,” de Kerchove says. “Someone might say they are going to the Turkish border to work in a refugee camp but from there they may join the Free Syrian Army and, step by step, join a more radical group because these are the ones with the best weapons and most of the money coming from the Gulf. How do you collect the evidence?”

De Kerchove says engagement and co-operation with neighbouring countries, particularly Turkey, is crucial. He is pushing for more rigorous surveillance of social media which is often used for recruitment purposes. He is also calling for the monitoring of personal travel data.

Also key is the role of influential figures within Muslim communities. Senior figures in Ireland have attempted to discourage young men here from going to fight in Syria, particularly after a Navan teenager was killed there earlier this year.

“It is much stronger and convincing . . . than us telling them not to go,” de Kerchove says.