Thieves broke into Paris' Louvre museum on Sunday, using a crane and smashing an upstairs window and stealing priceless jewelry from an area that houses the French crown jewels before escaping on motorbikes, the French government said.

The robbery is likely to raise questions about security at the museum, where officials had already sounded the alarm about lack of investment at a world-famous site, home to artworks such as the Mona Lisa, that welcomed 8.7 million visitors in 2024.

The thieves struck at about 9.30 a.m. (0730 GMT) when the museum had already opened its doors to the public, and entered the Galerie d'Apollon building, the Interior Ministry said in a statement.

The robbery took around four minutes, Culture Minister Rachida Dati told TF1, and was carried out by professionals.

"We saw some footage: they don't target people, they enter calmly in four minutes, smash display cases, take their loot, and leave. No violence, very professional," Dati said on TF1.

She said one piece of jewelry had been recovered outside the museum, apparently dropped as the thieves made their escape.

Dati declined to say what the item was, but newspaper Le Parisien said it was believed to be the crown of Napoleon III's wife, Empress Eugénie. The item was broken, the newspaper 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.

"Ideally, the perpetrators would realise the gravity of their crime and the dimension they've entered into, and return the items, since the jewels are completely unsellable," Giquello said.

"We’re touching on the history of France ... It’s a completely incomprehensible act, because the perpetrators will have every police force in the world on their tail, and in my opinion, they’ve put themselves in a very precarious situation," he added.