Urgent measures to prevent young people from going to wage jihad in Syria are to be unveiled by the government in Paris amid fears that its nationals were among the captors of four French hostages.

The measures include a plan to stop minors leaving France without parental consent, increased surveillance of Islamist websites, and a system to encourage parents to signal suspicious behaviour in their children.

Following the release on Saturday of four French journalists held in Syria for 10 months, the foreign minister, Laurent Fabius, revealed that some of their jailers had spoken French, apparently as a native language.

Briefing RTL radio on the plans, he said "about 500" French citizens were believed to be fighting with jihadi groups in Syria. "The numbers are rising, and they are becoming younger," he said.

Fabius said online surveillance would be intensified so that young people in danger of being radicalised could be identified early. As soon as someone starts looking at an Islamist website, they will be flagged up and monitored, he added. .

A special unit will also be established so that parents who suspect their children are being radicalised can get help before it is too late. Psychologists, social services and experts on Islamist groups will be on hand to answer the questions of young people attracted to the romantic images portrayed by Jihadi websites.

"The message we need to get across is that there's no glory out there, only death," an intelligence source said.

"The big Jihadi groups speak French to French people," said Dounia Bouzar, an expert on fighting the radicalisation of young Muslims. "One of the main 'gurus' has posted videos on the internet in which he speaks French. It's not surprising that so many young French and Belgian people are being hooked," she told Le Figaro.

The government is also to re-establish a rule obliging minors to obtain parental permission before they can leave the country, and says it will work with other European countries to step up border surveillance. It plans to work more closely with the Turkish authorities, as most would-be jihadis travel to Syria via Turkey.

It is not just young men and boys who are at risk. A 14-year-old girl from Lyon sent a text message to her parents in February saying she was en route for Syria, but was intercepted by police as she tried to board a flight to Istanbul.