Seven men suspected of fighting in Syria have been placed under formal investigation following their arrest in Strasbourg, said a judicial source on Saturday, weeks after France unveiled policies to prevent French Muslims from becoming radicalized.

Confirming a report in Le Monde newspaper, the source said the men aged 23 to 25, who were arrested in the eastern French city on Tuesday, were under investigation for criminal conspiracy in relation with a terrorist undertaking.

They were suspected of traveling to Syria late last year to join the uprising against Syrian President Bashar al-Assad.

European governments calculate several thousand of their citizens have gone to Syria since the war began three years ago and are worried about the security risk posed by youths radicalized by the conflict.

France, which estimates around 285 of its citizens are currently fighting in Syria among the rebels' ranks, last month launched a plan to prevent its citizens joining the civil war and becoming a threat to their home country.

Under the new policies, French nationals who return from Syria could face charges of being part of a terrorist organization, while minors could be prevented from leaving France without parental consent.

It also launched a hotline for parents concerned about their children's behavior.