Terrorism probe under way after gas cylinders found in Paris car

French police were holding a couple with links to militant Islam on Wednesday after gas cylinders were found in a car near Paris's Notre Dame cathedral, sources close to the investigation said.

The Peugeot 607 was discovered abandoned, with its headlights flashing, at the weekend in a part of the city that is hugely popular with tourists, the sources said.

Anti-terror investigators have launched a probe and the man and woman, both known to security services, were arrested Tuesday on a motorway in southern France.

The car's owner, known to authorities for his Islamist preaching, was released on Tuesday evening.

His daughter, whom he described to investigators as radicalised, was being sought by authorities on Wednesday, a police source said.

France remains on high alert after a string of attacks, including last November's coordinated Islamic State (IS) group assaults in Paris by gunmen and suicide bombers who killed 130 people.

A bar employee working near Notre Dame raised the alert on Sunday after noticing a gas cylinder on the back seat of the car, which had no number plates, a police source said.

That cylinder was found to be empty but five full cylinders were found in the boot of the car.

No detonators were found, police said.

Photographs of the metallic silver-coloured car after it was discovered showed its boot open and the gas cannisters placed on the ground in a quiet side street opposite the cathedral. 

'Parked for two hours'

Florence Berthout, the mayor of the district where the vehicle was found, complained in a letter to the Paris police chief that the car was "illegally parked for over two hours, despite several telephone calls to police headquarters".

The comment comes after authorities faced heavy criticism for security lapses in July, when 86 people were killed by an Algerian national ploughing a truck into a crowd celebrating Bastille Day in the resort of Nice.

The IS said the driver, Mohamed Lahouaiej Bouhlel, was one of its followers. 

Less than two weeks later, two young militants murdered a priest near the northern city of Rouen.

Interior Minister Bernard Cazeneuve said the latest arrests came at a time of heightened vigilance for potential attacks, adding that "the intentions of those arrested" were as yet unknown.

Notre Dame, renowned for its flying buttresses, stained-glass windows and gargoyles, is one of Paris's most popular landmarks, attracting 13 million visitors each year.

The head of France's DGSI domestic intelligence service, Patrick Calvar, warned in May of a "new form of attack" in which explosive devices would be left near sites that attract large crowds.

Such attacks would create large numbers of victims without sacrificing suicide bombers, he told a parliamentary committee.

French security services are particularly worried about the danger posed by militants returning from Syria after fighting with IS forces, with 700 French nationals still in that country, according to France's top prosecutor.