France’s interior minister revealed Thursday that hundreds of homegrown extremist militants were signing up to fight in Syria and warned they could pose a security threat when they come back.

France's interior minister revealed Thursday that hundreds of homegrown extremist militants were signing up to fight in Syria and warned they could pose a security threat when they come back.

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

Most of them were young men, often with a delinquent past, who had become radicalized, 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, there are up to 10,000 militants from all over the world currently fighting in Syria on the side of insurgent groups.

"If the extremists are not able to set up their state in Syria, they'll come back disappointed," Marc Trevidic, France's top anti-terrorism judge, was quoted as saying earlier this week.

Valls has previously warned that there are "several dozen, perhaps several hundred, potential extremists in our country" and described their presence as a ticking time bomb.

Citing intelligence reports, Valls said there were more than 130 French nationals or residents currently fighting in Syria, about 50 who had returned home, some 40 who were in transit areas and around a 100 who were likely to travel to Syria.