Woman close to French attack suspect put in custody

French investigators have placed in custody a woman close to the suspect gunman behind Friday's terror attack who killed three people and injured 16 others in southern France, Paris prosecutor Francois Molins said.

"A person close to him, who shared his life" was taken into custody for criminal association in relation with a terrorist attack, Molins told a press conference in Carcassone, without elaborating.

The dead gunman was identified as Redouane Lakdim, at 25.

Molins said Lakdim stormed a supermarket in Trebes, shouting the religious slogan "Allahu Akbar" and telling about 50 people who were inside the supermarket that he was "a soldier of the Islamic State" and was seeking "to liberate brothers".

According to BFMTV news channel, the 25-year-old man had asked for the release of Salah Abdeslam, the sole surviving suspect of Paris shootings and explosions that left 130 victims in 2015.

Known to police for petty crimes, Lakdim had been on the terror watch list since 2014 for links to Salafist movements, Molins added.

"The monitoring in 2016 and 2017 did not reveal any apparent signs that could suggest he would pass to an eventual terrorist act," he said.

Before attack in the supermarket, Lakdim had killed one person and wounded another before stealing a car. Following that, he opened fire on police officers while they were jogging in Carcassone, injuring one person. Then he headed to Trebes and killed two people in the supermarket where he held hostages before being shot dead.

The Islamic State claimed responsibility of the attack. The claim was being checked by French security services, President Emmanuel Macron said after chairing a crisis meeting with ministers and security officials.

"I want to tell the nation tonight of my absolute determination in leading this fight," Macron said.

"I urge our fellow French citizens to remain aware of the terrorist threat, but also to remember the force and resistance our people have demonstrated each and every time they were attacked," he added.

France has become a major target of frequent terrorist attacks over the past several years.

A wave of attacks, claimed by the Islamic State, had broken several times the calm at home with the bloodiest was in Paris where a series of explosives and shootings left 130 victims in November of 2015.

In October 2017, Macron signed anti-terrorism law which he said was necessary to muscle security at home to combat high terrorism menace.

The bill enshrines emergency security rules into ordinary law that allow police will have more power to search, arrest without judge approval and restrict people movements and gathering.