* First wave of Syria jihadists leading way for new recruits
* Risk of attacks on home soil is mid-term - anti-terror judge
* Legal repression is not sufficient to stem exodus
By Nicholas Vinocur and Chine Labbé
PARIS, Feb 12 (Reuters) - Growing numbers of French youths are leaving to fight with Islamist rebel groups in Syria as a first wave of volunteers returns home and steps up a recruitment drive for jihad, a top anti-terror judge said on Wednesday.
Several Western European countries are struggling to contain a trend that has seen as many as 2,000 youths travel to Syria to combat President Bashar al-Assad's government since a rebellion started nearly three years ago.
With Europe's largest Muslim population, France has become a major centre for recruitment and judicial authorities are grappling with a fresh wave of adolescent volunteers, some of whom are as young as 15 years old.
"Major events like the use of chemical gases have inspired many people" to join Islamist groups in Syria, said Marc Trevidic, an investigating judge specialised in counter-terrorism and Islamist radicalisation.
"But there has also been a very obvious accelerator, which is that the first generation of recruits have come back home to fetch their buddies," he told Reuters.
President Francois Hollande warned about the issue in January when he said that some 700 people had left France for Syria. The number appeared to contradict an earlier estimate from his interior minister of around 250.
Trevidic said the discrepancy was between the number of French citizens known to be fighting in Syria and the larger number of people passing through France on their way to Turkey, which shares a border with Syria.
"There was an explosion when that first generation came home... As soon as they get back, they are itching to leave again," he said, adding that recent waves included female recruits who left to marry jihadists.
'TREAT THE CAUSES'
A French court last month placed two youths, one of them just 15, under formal investigation on suspicion of planning terror acts after they were arrested in Turkey on their way to Syria, a move their lawyer criticised as legally unsound.
Trevidic said he was in favour of stopping recruits who plan to join Islamist groups in Syria from leaving France. Those who have returned are questioned by anti-terror security agents and kept under surveillance if they had joined hardline groups or had combat experience, he said.
While he did not see an immediate risk of terror acts in France, he said the prospect would become imminent in the medium term as combatants grew discouraged at the idea of an Islamic state ever being founded in Syria.
However, Trevidic said that legal repression alone was not enough to stem the exodus as parents who suspect their children are becoming radicalised currently have no option other than to alert police and internal security services.
"Things are getting worse, so we need to treat the causes," he said, referring to civil society structures to help detect and dissuade radicalised individuals.
He criticised the idea of stripping Syrian combat volunteers of their citizenship, as the British government has recommended, because that would lead to creating a stateless group more prone to attacking Western governments.
"As long as Bashar's regime is in place, as long as the jihadist groups think there is a chance of overthrowing it and creating an Islamist state... they will focus their strength in Syria," he said. "It's when they are disappointed that things become more dangerous." (Writing By Nicholas Vinocur; Editing by Tom Heneghan)