Paris (AFP) - French police investigating a car containing six gas cylinders in Paris have arrested three female suspects said to have been planning fresh attacks, shooting one of them in the process.

A police officer also suffered a knife wound during the arrest late Thursday of the three women -- aged 39, 23 and 19 -- who were preparing "new violent ... and imminent actions", according to Interior Minister Bernard Cazeneuve.

The woman shot, a daughter of the car's owner, had sworn allegiance to the Islamic State group, according to a source close to the investigation.

A local resident who saw the women just before their arrest in Boussy-Saint-Antoine, south of Paris, said they "seemed anxious, looking around them all the time".

When confronted, "one young women pulled out a knife and stabbed a police officer at stomach level", Joachim Fortes Sanchez, 21, told AFP. "It all happened very fast".

Four people -- two brothers and their girlfriends -- were already in custody over the car, found just a few hundred metres (feet) from the Notre Dame cathedral in central Paris, an area always thronged with tourists.

The first couple arrested, a 34-year-old man and a 29-year-old woman, have been held since Tuesday and are known to the security services for links to radical Islamists.

Police arrested the man's brother and his girlfriend, both aged 26, late Wednesday.

Authorities had said earlier they were searching for the two daughters of the owner of the grey Peugeot 607 abandoned on Sunday near Notre Dame, which draws millions of visitors every year.

- Plots foiled -

A bar employee working near Notre Dame had first raised the alert on Sunday after noticing a gas cylinder on the back seat of the parked car, police said.

The car had no number plates and its hazard lights were flashing.

Although the cylinder on the back seat was empty, five full cylinders were discovered in the boot.

Three bottles of diesel fuel were also discovered in the vehicle, but police found no detonators.

Story continues

"If it was an attack plot, the method was very strange," a police source said Thursday.

France is on high alert following a string of jihadist attacks, including last November's coordinated bloodshed in which Islamic State extremists killed 130 people.

Speaking on Thursday, President Francois Hollande referred to attack plots that have been foiled "in recent days", without elaborating.

Cazeneuve on Friday told French daily La Presse that 260 people have been arrested in connection with terrorist networks or operations since the beginning of the year.

- 'Strange method' -

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

The discovery followed a deadly summer in France in which 86 people were killed when a truck ploughed into a Bastille Day crowd in the southern resort of Nice.

IS said the truck was driven by one of its followers.

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

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

French security services are particularly worried about the danger posed by extremists returning from Syria after fighting with IS forces.

Around 700 French nationals are still in Syria, France's top prosecutor said last week.