Police are hunting a gang of armed thieves who staged a bold daytime art robbery, walking out of a Riviera museum with paintings by the Impressionists Claude Monet and Alfred Sisley and two by the Flemish master Jan Breughel the Elder.

Investigators said that the gang of five knew exactly which paintings they were seeking when they arrived at the Jules Chéret museum in the heart of Nice on Sunday lunchtime, at a moment when staff were reduced to four. The day was the one Sunday each month on which entry is free.

Two wore motorcycle helmets with visors, two wore surgical masks and one wore heavy dark glasses. Two of the men wore white chemical hazard suits. One stayed in the lobby, aiming a gun at staff, while the others held up the two guards on the first floor, where the paintings were hanging.

At gunpoint, they ordered staff to lie on the floor while they removed the paintings from their frames and put them in black bags. Only about six visitors were in the Genoese-style mansion at the time of the robbery.

Police believe that the men were working to order as the stolen paintings — Monet’s Cliffs near Dieppe (1897), Sisley’s Lane of Poplars at Moret-sur-Loing(1888), and Breughel’s Allegory of Water and Allegory of Earth — are too well known to sell.

“The robbers tried to carry off a second Sisley, but they dropped it and broke the frame after finding that it was too heavy or too cumbersome,” Patricia Grimaud, deputy curator at the museum, said. The two works by Breughel (1568-1625) were also signed by Hendrick van Balen, a fellow artist.

The Monet and the Sisley, on permanent loan from the Paris Musée d’Orsay, had survived earlier theft. In 1998 Jean Forneris, at that time the curator of the museum, had the same two pieces stolen by two accomplices. They were recovered a week later on a boat in a nearby town. Forneris was sentenced to 18 months in prison in 2002. The Sisley was also stolen in 1978 when on loan to a Marseilles museum and found a few days later.

The Nice authorities are reviewing security arrangements at the museum, which is the third-most-visited site in the city. But officials said that there was little that could be done to stop determined armed robbers. The hold-up was the second big art theft in France this year. In February two paintings and a drawing by Pablo Picasso were stolen from the Paris flat of one of his granddaughters. The works have not been found.

Police said that the thieves could be aiming to ransom the stolen works from the insurance firm company that would have to pay out for the robbery.

Christine Albanel, the Culture Minister, expressed indignation and sadness over the theft and urged the thieves not to harm the works. She congratulated the museum staff on their calm reaction to the hold-up, in which no one was hurt.

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