PARIS, JULY 6 -- Declaring a crisis in France, the head of the Louvre museum announced today that a dozen Egyptian artifacts have been missing since Wednesday, the same day three paintings were stolen from French museums.

"We noticed they were missing on Wednesday. This morning we decided that it was a theft," said Michel Laclotte of a group of 2nd- and 3rd-century gold necklaces, bracelets and earrings.

The Roman-period jewelry from Egypt had been locked in a glass display case. Though each piece is worth only about $1,000, their theft demonstrated for the fourth time in a week the vulnerability of French museums.

On Wednesday, a small Renoir painting was cut from its frame in the Louvre, apparently in broad daylight in a guarded area. The same afternoon, a painting by 19th-century artist Ernest Hebert was stolen from the Hebert Museum and a painting by an artist of the same period, Paul Huet, was taken from the wall at the Carnavalet Museum. None of the pieces was insured.

Jacques Sallois, director of French national museums, closed five small museums in Paris this week because of security problems there. He said the painting thefts appeared the work of an "able imbecile."

"Certainly this is a crisis," said Laclotte, talking to reporters in a reception room beside I.M. Pei's glass pyramid in the courtyard of the Louvre. "We will use this crisis to increase our security measures." He said the Louvre would increase its security budget next year by 10 million francs, approximately $1.8 million.

Laclotte would not specify what security measures would be taken, but said the Louvre would undertake a fundamental revision of its security policy. The museum is watched by 550 guards throughout and by an electronic system in some parts. The Renoir had been attached to an alarm that did not go off because the thief did not remove the frame.

Louvre officials said they would hire a "super-specialist" in security and that police would review the museums to make suggestions on security. They would not discuss specifics for fear of giving clues to potential thieves.

The thefts have clearly embarrassed museum administrators, who normally avoid speaking to the press and are now being ridiculed in the media as running "self-service" museums.

The latest thefts have highlighted the difficulties of keeping artwork safe in a period when impressionist works fetch millions of dollars in auction houses. While well-known stolen artwork is virtually worthless except for ransom, lesser-known works such as the 13 1/2-by-10 1/2-inch Renoir, "Portrait of a Seated Woman," could potentially be resold, art dealers said.

Museum officials lamented that security is particularly difficult during the summer as thousands of tourists flock to Paris to visit such works as the "Mona Lisa" and "Venus de Milo."

"It is like living in a bank with its coffers open," said Paul Salmona, a spokesman for the Louvre. "The problem is every measure we take for security hurts the quality of conditions for visitors. And it is not possible for us to place a guard in front of every painting." The Louvre houses some 6,000 paintings, 150,000 sketches and hundreds of thousands of art objects.

Laclotte said the Louvre would close several rooms that it deemed too difficult to guard until improved security measures were in place. But even selecting which rooms to close would be problematic, he said, because the Louvre -- a palace for Louis XIV -- has so many enormous rooms full of priceless art.

Another alternative would be to remove artwork from public view and guard it in storage. But that may not solve the problems either; a government inventory of art thefts over the past decade was published in June and showed that 57 percent of stolen art is taken from storage.