Four robbers looted 17.5 million euros in jewellery and watches from a boutique in the French resort town of Cannes just days before the start of its star-studded film festival, sources said Thursday.

The Riviera town that hosts the red carpet extravaganza has become a key target for thieves carrying out film-worthy heists at a time of year when top jewellers have their best pieces in stock to adorn the globe's hottest celebrities.

The threat of such thefts and jitters four months after militant attacks in Paris left 17 dead have led authorities to boost security for the glittering festival which sees the population of the small town triple to 210,000 people.

The latest robbery came on Tuesday at the Cartier boutique on the beachfront promenade known as the Croisette.

One of the robbers wore an old-man mask and carried a pistol as he barged into the store, letting in two accomplices with scarves around their faces, while a fourth man stood guard outside, said sources close to the investigation.

Two employees and a shopper were in the store at the time.

The robbers stuffed the jewellery and watches into a leather satchel and jewellery bag. In their rush to escape, a number of expensive watches fell to the pavement and were recovered by the store.

It was not known whether the stolen items were brought in especially for the film festival.

The thieves made off in a Mercedes that was later found burned out in a residential part of town. The car had been stolen in March.

Police are using surveillance cameras from the store in a bid to identify the thieves.

The large Cartier store sits next to the luxury Carlton hotel, which was also hit by a jewellery theft in July 2013 while it was hosting an exhibition and sale by Israeli brand Leviev.

On that occasion, the thief helped himself to over 140 million euros' worth of items -- including diamond-encrusted jewellery -- after breaking into the hotel through a door on the ground floor.

The man with a semi-automatic pistol, his head covered in a cloth, walked into an exhibition being held in a wing of the hotel with direct access to the street.

The robber cleared the display cabinets before simply melting into the crowd on the Croisette -- lined by some 70 luxury stores -- and has never been caught.

More people, more wealth

That robbery was part of a string of robberies in 2013, many of which remain unsolved.

Experts say it is relatively easy for robbers to smuggle jewels out of France, find a corrupt polisher to change its shape and resell it.

Also in 2013 a thief broke into the hotel room of an employee of Swiss jewellers Chopard and made off with a $1.4 million haul while another made off with a $1.9 million diamond necklace during a party at the exclusive Hotel du Cap-Eden-Roc near Cannes.

"There are more people and more wealth, thieves come especially for the event," said Cannes police commissioner Philippe Jos.

An additional 500 officers are being sent to Cannes to secure this year's festival -- most of whom will be patrolling in civilian clothes to remain discreet.