Paris’s Louvre museum has remained closed after historic jewels were stolen in a brazen daylight heist, as experts admitted many of the precious items may never be recovered.

Thieves in balaclavas used a crane to smash an upstairs window early on Sunday, then stole priceless objects from an area that houses the French crown jewels before fleeing on motorbikes.

On Monday, police and art experts said only a small pool of criminals would be capable of such an audacious robbery, and may already be known to police.

They said crime gangs in Europe were increasingly robbing valuable jewels and gold from cash-needy museums like the Louvre.

While the thieves are often caught, it can be a struggle to return the priceless priceless goods. The objects could be quickly broken down into component parts and sold on, experts warned.

“If I steal a Van Gogh, it’s a Van Gogh. I can’t dispose of it through any other channel than an illicit art market,” said Marc Balcells, a Barcelona-based expert in crimes against cultural heritage.

“But when I am stealing … jewellery, I can move it through an illicit market as precious stones.”

The thieves struck about 9.30am local time, when the museum had already opened its doors to the public. They entered the Galerie d’Apollon building, Paris Prosecutor Laure Beccuau said on BFM TV.

The robbery took between six to seven minutes. It involved four people who were unarmed, but who threatened the guards with angle grinders, she said.

Footage circulating on X, and claiming to be shot by a Louvre visitor, appears to show one of the gang breaking into a glass case to steal some of the jewels.

In all, they targeted nine objects, getting away with eight. The last item – the crown of Napoleon III’s wife, Empress Eugenie – was lost during their escape, Beccuau said.

“It’s worth several tens of millions of euros – just this crown. And it’s not, in my opinion, the most important item,” Drouot auction house President Alexandre Giquello told Reuters.

The brazen heist of crown jewels from the Louvre, the world’s most visited museum, has been decried by some as a national humiliation and sparked security checks across France’s multitude of cultural sites.

“If you target the Louvre, the most important museum in the world, and then get away with the French crown jewels, something was wrong with security,” art investigator Arthur Brand said.

“It’s one of the biggest manhunts in French history.”

Officials at the Louvre, home to artworks such as the Mona Lisa, had already sounded the alarm about a lack of investment.

And at least four French museums have been robbed in the past two months, including gold stolen from the Natural History Museum in Paris, according to media reports.

Christopher Marinello, founder of Art Recovery International, which tracks stolen art, said such museum heists were on the rise across Europe and further afield. He cited cases in the Netherlands, France, Egypt.

“If you have jewels or gold in your collections, you need to be worried,” Marinello said.

Paris prosecutors have entrusted the investigation to a specialised French police unit known as the BRB, which deals with high-profile robberies.

Former cop Pascal Szkudlara, who served in the unit, said the BRB handled the 2016 Kim Kardashian probe, when Paris thieves stole her $US4 million ($A6.2 million) engagement ring, as well as a spate of kidnappings of wealthy crypto bosses.

He said the BRB had about 100 agents, with more than a dozen specialised in museum thefts. Investigators will look at video footage, telephone records, and forensic evidence, while informants will also be activated.

“They can have teams working on it 24/7 and for a long period,” Szkudlara said, expressing “100 per cent” confidence the Louvre thieves would be caught.

Police would be poring over security footage going back weeks, looking to identify suspicious people casing out the joint, Brand said.

Corinne Chartrelle, an officer who previously worked at the French Police’s Central Office for the Fight against Trafficking in Cultural Property, said the jewels could feasibly end up in a global diamond centre like Antwerp where there “are probably people who aren’t too concerned about the origin of the items”.

The diamonds could also be cut into smaller stones and the gold melted down, leaving buyers unaware of their provenance.

If the thieves felt the net closing, they could dispose of or destroy the loot altogether. Police were clearly in a race against time.

“Once they’re been cut into smaller jewels, the deed is done. It’s over. We’ll never see these pieces again intact,” Marinello said.

“It’s a very small percentage, recovering stolen artworks. When it comes to jewellery, that percentage is even less.”

-with AAP