* About 700 French nationals involved in Syria civil war
* Raft of policies to be unveiled Wednesday
* Measures to stop minors travelling, monitor Islamist sites (Recasts with interior minister, background, analyst)
By John Irish and Marine Pennetier
PARIS, April 22 (Reuters) - France's interior minister on Tuesday unveiled a raft of policies to stop its citizens joining the Syrian civil war, aiming to prevent young French Muslims becoming radicalised and posing a threat to their home country.
France, which has been a staunch opponent of Syrian President Bashar al-Assad, estimates the number of its nationals directly or indirectly involved in the Syrian conflict at about 700 of which a third are fighting against the government.
President Francois Hollande has made clamping down on violent cells and self-radicalized operators planning domestic attacks a priority since a Toulouse-based al Qaeda-inspired gunman, Mohamed Merah, shot dead seven people in March 2012.
But with the Syrian conflict entering its fourth year, the government has increasingly come under criticism for failing to stop its nationals - some as young as 15 - from heading to Syria.
"France will take all measures to dissuade, prevent and punish those who are tempted to fight where they have no reason to be," President Francois Hollande told reporters on Tuesday.
Highlighting Paris' concerns, at the weekend, four French journalists who returned from Syria after being held by an al Qaeda-linked group, said some of their captors had been francophone.
Speaking on France 2 television before officially unveiling the measures to cabinet on Wednesday, Interior Minister Bernard Cazeneuve said the new measures could go as far as stripping people of French nationality along the lines of new British legislation introduced last year.
Parents will be encouraged to signal suspicious behaviour in their children through a dedicated hotline after which officials will immediately be sent to assess the situation.
"French Islam is not radical...what we need to do is prevent this behaviour. Minors and teenagers are often fragile and can fall into the hands of preachers of hate and recruiters," Cazeneuve said.
In total some 20 measures will be presented, with those not requiring new legislation being implemented in the coming days.
Cazeneuve also said minors would be prevented from leaving France without parental consent, and that the names of those identified as wanting to leave for jihad would be signalled to the European Union's 27 other members.
French nationals who returned from Syria could now also automatically face criminal charges for being part of a terrorist organisation, he said.
"This is a comprehensive plan to fight a phenomenon that is in sharp progression," a government source said.
"The idea is to deal with the problem from when someone is in their room watching jihadi videos to the moment when they are taking the bus ... to the Turkish-Syrian border."
SOCIAL MEDIA BOOM
France - which has Europe's largest Muslim population at about 5 million - has had broad success at dodging attacks in large part due to its water-tight security apparatus and some of Europe's toughest anti-terror laws, although analysts say they need to be adapted in light of the social media boom.
Cazeneuve said the new measures would enhance surveillance of Islamist websites that recruit fighters and aim to block assets of those behind them. He said Paris would also push European partners to close down extremist sites.
Critics say Paris has until now turned a blind eye to its nationals fighting Assad, preferring that they be active in Syria than at home in France.
"Today, this strategy has led to the authorities being overwhelmed," said David Thomson, author of The French Jihadists, a book published in January that traced the path of 19 jihadists who joined al Qaeda affiliates in Syria.
"They didn't take this threat seriously and had the same mindset as the 1990s. The ideology of these youngsters is built on being anti-authority. In their eyes, the authorities are non-believers, so what they decide has to be fought." (Editing by Andrew Callus)