Masked raiders have stolen four paintings in a daylight raid on an art gallery in Nice in the south of France.
Police said the stolen paintings were worth about 1m euros
Police said five men made off with two works by Flemish artist Brueghel and two paintings by the impressionists Sisley and Monet on Sunday.
Officials at the Musee des Beaux-Arts said the works were "invaluable".
Two of them were stolen from the same museum nine years ago, but were recovered within a week. The then curator was later jailed for theft.
In the latest raid, the men, wearing masks and jumpsuits, entered the gallery in the heart of Nice when entry was free for the public.
They threatened members of staff before removing the paintings and escaping - two on a motorcycle and three in a car.
Allegory of Earth by Jan Brueghel was among the works stolen
"One of the staff members told me that the men told him to lie on the floor as they put the paintings in bags," the museum's assistant curator Monique Bailet said.
"They wanted to take a fifth but could not do so."
The stolen paintings include Claude Monet's Cliffs Near Dieppe, from 1897, and Alfred Sisley's The Lane of Poplars at Moret, which dates from 1890.
The two stolen works by Jan Brueghel, a Flemish Baroque-era painter, were Allegory of Water and Allegory of Earth.
Police in Nice said the combined value of the four paintings was about 1m euros (£670,000).
They believe the works were stolen to order, as they are too well-known to be sold on the open market.