A huge manhunt is under way for surviving members and accomplices of the Islamist group that killed 129 people in Paris on Friday night.
Police have named Salah Abdeslam, 26, as a key suspect. He was reportedly stopped by officers in the wake of the attacks – but then let go.
Meanwhile, French aircraft have attacked the Syrian stronghold of IS.
Latest reports from France say police investigating the assaults have carried out a series of raids.
French media organisations say major police operations have taken place in Bobigny, a suburb of Paris; Jeumont, close to the French border with Belgium; and in the southern city of Toulouse.
The Islamic State group has said it was behind Friday’s gun and bomb attacks on Parisian bars and restaurants, a concert hall and the Stade de France, the country’s main sports stadium.
Seven attackers died in the assault on the French capital, most of them after detonating suicide belts.
Salah Abdeslam, the man named by police as a key suspect, is said to have rented a VW Polo car that was found near the Bataclan concert hall, where 89 people died, and believed to have been used by attackers.
On Saturday he was in a vehicle near the Belgian border which was stopped by police, officials said. He was reportedly questioned, had his papers checked and was then released along with two other men who were also in the car.
It is unclear whether the French authorities had matched the VW Polo found at the Bataclan venue to him at the time he was stopped.
Police have described Salah Abdeslam as dangerous, and warned people not to approach him.
He is one of three Belgium-based brothers linked to Friday’s attacks, officials say.
Another, Brahim Abdeslam, 31, is said to have been an attacker who blew himself up outside a bar on the Boulevard Voltaire near the Bataclan, wounding one person.
A third brother, Mohammed, was reportedly arrested in the Brussels neighbourhood of Molenbeek while returning from Paris and remains in custody. Belgian police say they have made a total of seven arrests.
Officials said the brothers were traced following the discovery of two rental cars registered in Belgium – the VW Polo rented by Salah Abdeslam, and a Seat containing Kalashnikov automatic rifles that was found abandoned in the Paris suburb of Montreuil.
One of three attackers who died at the Stade de France after detonating suicide belts has been named in media reports as Bilal Hadfi, who was aged about 20 and also lived in Belgium.