French police arrested four people on Monday suspected of having helped recruit volunteers for radical Islamist networks in Syria, Interior Minister Bernard Cazeneuve said.
European governments increasingly worry that citizens, often from a Muslim immigrant background, going to fight in Syria's civil war will import Islamist militancy upon their return, Reuters reported.
Monday's arrests came a day after prosecutors said a 29-year-old Frenchman thought to have returned recently from fighting with Islamist insurgents in Syria had been arrested over the May 24 killing of three people at Brussels' Jewish Museum.
However, there was no suggestion of a direct link between the arrest of Mehdi Nemmouche after a random check in the southern city of Marseille on Friday and the four new ones.
"There have been four arrests this morning," Cazeneuve told French radio Europe 1, referring to the police action in the Paris region and the south of France. "We are acting everywhere. There will be no respite in the fight against terrorists."
Nemmouche was detained in Marseille after he was found to be carrying a Kalashnikov rifle, another gun and ammunition similar to those used in the museum shooting that left an Israeli couple and a French woman dead, French and Belgian prosecutors said.
Marine Le Pen, whose anti-immigrant, Euro-sceptic National Front party finished first in France in European Parliament elections just over a week ago, said the Nemmouche episode showed "the stupifying naivety of the government".
Although Nemmouche's ethnic background has not been publicly confirmed, Le Pen made the link to immigration into France from Muslim countries.
"We must put a stop to this mass immigration," she told France Info radio, calling for improved surveillance, tougher border controls and special prison units for jihadist suspects.
Referring to estimates that some 700 French nationals had gone to fight in Syria, Roger Cuikerman, head of France's CRIF Jewish association, said he feared "they would become 700 time bombs when they return.
"The DGSE [external intelligence service] should probably get more resources to be able to follow the tracks of these people very closely," he told RTL radio.
The arrest of Nemmouche brought back painful memories in France of the shooting deaths of four Jews in 2012 at a school in the southwest of France by Mohamed Merah, a French citizen of Algerian heritage inspired by al Qaeda.
It also came days before the French parliament is due to study a disputed government-backed penal reform bill under which France will in some cases look to propose sentences other than jail time for offenders.