Manuel Valls, French Interior minister said on Thursday that the number is worrying for French national security.

According to Valls, hundreds of home-grown militants were signing up to fight in Syria, and warned they could pose a security threat when they return to France.

More than 300 French nationals or residents are either currently fighting in Syria, are known to have plans to go and fight, or have recently returned from Syria, the minister, Manuel Valls, told France Inter radio.

Most of them were young men with a delinquent past who had become radicalised, he said.

"This is a phenomenon which worries me because they represent a potential danger when they return to our soil," Valls said. "We have to be extremely attentive."

According to British defense consultancy IHS Jane's, up to 10,000 militants from all over the world are fighting alongside foreign-backed terrorists in Syria.

At least one French national has been killed fighting in Syria. A 22-year-old French national from Toulouse was killed in a clash with government troops in August.

MRKD/NJF