PARIS -- About 15 gunmen ambushed two vans carrying jewels worth millions on a French highway in the dead of night, ejecting their drivers and speeding off into the Burgundy countryside, police said. It was the latest of several big jewelry heists in France.

Unusually this time, the attackers chose a moving target instead of one of France's many high-end jewelry boutiques. To pull it off, experts said, the gang must have been tightly organized and well-informed, possibly thanks to an inside source with knowledge of the vans' movements.

The assailants and the jewels remained missing Wednesday evening, even after gendarmes and other authorities spent hours combing the forests and towns southeast of Paris around the scene of the overnight attack.

The vans were slowing down to approach a tollbooth on the A6 highway connecting Paris and Lyon when four cars apparently surrounded them and forced them to stop, a security official said. No one was injured, and the drivers of the two vans were left at the scene unharmed, a police official said.

The officials spoke on condition of anonymity because they were not authorized to be publicly named.

The perpetrators then escaped in four cars and the two vans, which police later found burned and abandoned about 50 kilometres (30 miles) away, the police official said. Forensic investigators in white suits examined the area around the charred vans Wednesday amid vineyards in the town of Quenne.

It's unclear where the attackers were heading. After hours of searches failed to locate them, authorities concentrated on their investigation into the attack, which was handed to the French police agency overseeing organized crime.

No suspects have been named.

Alain Bauer, a professor of criminology, said Wednesday's heist did not appear to fit the pattern of attacks by the Pink Panthers gang of jewel thieves because they "don't usually attack trucks." That network has been credited with a series of thefts that Interpol says have netted more than 330 million euros since 1999.

France itself sees about five or six jewelry heists a year, Bauer said.

"That's actually low, historically. Ten or 20 years ago, we had two or three times more," he said.

The number has diminished because French authorities have dismantled several jewelry theft networks. Today, Bauer said, most of the teams staging heists in France are coming from the Balkans or elsewhere in eastern Europe.

In November, two gunmen robbed a Cartier jewelry boutique in a tony, tourist-filled Paris neighbourhood. Police chased them across the Seine River and they took a hostage before surrendering.

Last month, eight people were convicted in connection with a spectacular 2008 jewel theft at a Harry Winston boutique in Paris, when three cross-dressing gunmen stole about $92 million in goods, aided by a security guard.

In 2013, southeast France was hit with a spate of jewelry thefts, including two in Cannes during the city's famed film festival. In one, a gunman walked into a jewel show at the Carlton International Hotel, stole $136 million in loot, and disappeared down a side street in one of the most lucrative jewelry heists ever. No one has been convicted in the case.

Major diamond heists in recent years have also targeted Antwerp in Belgium, Amsterdam, Milan and London.