More than 300 French nationals or residents are either currently fighting in Syria's civil war, 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 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."

France, which has the largest Muslim population in western Europe, has increased its monitoring of Islamic radicals since  Al-Qaeda-inspired gunman Mohamed Merah killed seven people in and around the southwestern city of Toulouse last year.

It subsequently emerged that Merah had spent time in Pakistan and Afghanistan and that French intelligence had been aware of his contacts with militants in those two countries.

According to British defence consultancy IHS Jane's, there are up to 10,000 jihadists from all over the world currently fighting in Syria on the side of rebels trying to overthrow President Bashar al-Assad, whose regime they want to replace with an Islamic state.

Experts in counter-terrorism fear that a chemical weapons attack near Damascus on August 21 could inspire more radicals to embark on jihad, or holy war, in Syria, increasing the numbers of a new generation of battle-hardened militants capable of wreaking havoc when they return to their home countries.

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

At least one French national has died fighting in Syria – a 22-year-old white convert to Islam from Toulouse only identified as Jean-Daniel, who was killed in a clash with government forces in August.

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

In October 2012, police shot dead the alleged ringleader of an Islamist cell suspected of carrying out a grenade attack on a Jewish grocery store in a Paris suburb the previous month.

A prosecutor branded that homegrown group of Islamist extremists as the biggest terror threat the country had faced since the Algerian-based GIA carried out a string of deadly bombings in the 1990s.

Islamist groups threatened to stage attacks in France as well as on French targets after Paris intervened in Mali early this year in reaction to advances made by Islamist groups who had seized control of the north of the country.

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.