A Paris travel tip for the über-rich: Think about keeping your valuables at home.

For the second time in two months, thieves - relying on surprise and precision timing - have gotten away with millions of dollars in jewels and left authorities with few clues but much embarrassment.

The latest targets were Qatari women in their 60s headed into Paris on Monday from Le Bourget airport, where private planes often land. Masked men stopped the Bentley carrying the women, sprayed them with tear gas and made off with no less than US$5.6 million in jewellery and other valuables.

One police source told the AFP news agency that "everything in the vehicle" was taken.

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The shocking incident came just two months after the reality TV icon and Instagram maven Kim Kardashian was tied up and robbed at gunpoint in an exclusive private hotel during Paris Fashion Week. In that episode, the thieves - whose identities remain unknown - stole a reported US$9 million of jewellery, including the 20-karat diamond ring given to Kardashian by her husband, the rapper Kanye West.

In recent years, there have been a number of similar smash-and-grab robberies in the same area where the Qatari women were attacked. Police say criminals favour the portion of highway around the lengthy so-called Landy Tunnel because of the steady flow of wealthy travellers who frequently pass between Le Bourget or the bigger Charles de Gaulle International Airport and the city. In a tight, contained environment, there is often traffic gridlock - plus plenty of exit routes where assailants can easily escape.

In February 2010, Christine Chernovetska, the daughter of Kiev's then-mayor, told Paris police that thieves stole $4.8 million euros in jewellery from her as she was stopped in traffic (although the details were later disputed). In August 2014, a troop of Kalashnikov-wielding men stole $330,000 worth of valuables from a convoy of cars belonging to a Saudi prince headed into Le Bourget. Likewise, in April 2015, three robbers shattered the window of a taxi carrying a Taiwanese art collector, stealing a handbag that contained $5.6 million of her jewellery.

Earlier this month, the French government pledged to install more surveillance cameras in portions of Paris where robbers often strike.