Paris, France (BBN)-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, reports BBC.
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, 26 – urgently sought by police
Mohammed Abdeslam – reportedly arrested in Belgium
Brahim Abdeslam, 31 – named as attacker who died near Bataclan concert hall
Ismail Omar Mostefai, 29, from near Paris – died in attack on Bataclan
Bilal Hadfi, 20 – named as attacker who died at Stade de France
Four other attackers died during the assaults in the city
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.
French Interior Minister Bernard Cazeneuve said Friday night’s attacks in Paris had been prepared “by a group of individuals based in Belgium” who had “benefited from accomplices in France”.
Belgium’s Premier Charles Michel said the Belgian authorities would crack down on Molenbeek, which has a reputation as being a haven for jihadists.
“I have noticed there is almost always a link to Molenbeek, that there is a gigantic problem there,” he said.
Brussels mayor Yvan Mayeur added: “We must stop being a base for those who make war in Europe.”
French media reports have named Bilal Hadfi, who also lived in Belgium, as one of three men who blew themselves up outside the Stade de France on Friday.
Another attacker to have been identified is 29-year-old Frenchman Ismail Omar Mostefai, who blew himself up at the Bataclan. He had a criminal record and had been flagged up as a possible Islamist extremist by French intelligence.
The authorities are also looking into the provenance of a Syrian passport found at the site of the Stade de France suicide bombing in the name of a man who arrived as a refugee in Greece before claiming asylum in Serbia.
French President Francois Hollande has said the attacks were an act of war and promised that his country’s reaction would be pitiless.
On Sunday, 10 fighter jets operating out of French bases in Jordan and the UAE dropped 20 guided bombs on four targets in Raqqa, the French defence ministry said.
The targets included a command centre, recruitment centre for jihadists, a munitions depot and a training camp for fighters, the ministry said.
The strikes were carried out in co-ordination with US forces.
France is currently marking a third and final day of national mourning. A state of emergency declared by President Hollande remains in force, and thousands of extra police and troops are on the streets of Paris.
There have been several security alerts and scenes of panic in Paris over the weekend, where people remain jittery following Friday’s bloodshed.
Bataclan concert venue, 50 Boulevard Voltaire, 11th district – 89 dead when stormed by gunmen, three of whom were killed; another gunman died nearby
La Belle Equipe, 92 rue de Charonne, 11th district – 19 dead in gun attacks
Le Carillon bar and Le Petit Cambodge restaurant at rue Alibert, 10th district – 15 dead in gun attacks
La Casa Nostra restaurant, 92 rue de la Fontaine au Roi, 11th district – five dead in gun attacks
Stade de France, St Denis, just north of Paris – explosions heard outside venue, three attackers and a bystander killed