LOS ANGELES — A dozen valuable paintings, including works by Marc Chagall, Diego Rivera and Emil Nolde, were stolen from the home of an elderly couple, and police issued an international alert to recover them.

A $200,000 reward was offered Tuesday for help in recovering the artwork, stolen Aug. 23 from a home in the San Fernando Valley. Police have notified auction houses, the FBI and Interpol in case someone tries to sell the paintings.

"These are world-class pieces," said Richard Rice, a senior consultant at the Gallerie Michael in Beverly Hills. "Every single one is museum caliber."

Police and art experts said it would be almost impossible to resell the paintings at anything near their true value. Stolen art is flagged on numerous Web sites and police databases.

"If you look at the last 10 years, a lot of the art that has been stolen that is this important has been returned to the owners because it is so difficult to resell," Rice said.

The theft occurred after a maid went to the store and left a service door unlocked, police said. In the hour or so the maid was gone, the paintings were taken from two adjacent rooms, detective Donald Hrycyk said.

The elderly couple who lives in the home were in a different part of the house at the time of the theft and did not hear an intruder. The husband is bedridden and the wife was sleeping.

Detectives have interviewed the maid, who has worked for the couple several years, and several other domestic helpers. No arrests have been made.

"Everybody is a suspect," Hrycyk said. "We are looking into the reasonableness of the maid's statement, and we are looking at the other people who have had access to the house."

Rice estimated the stolen paintings range in value from about $800,000 to as much as $4 million. He believes the most valuable was the Kees van Dongen portrait "Alicia Alanova," a 1933 oil of a woman in a hat.

The artworks were insured by the unidentified couple, who made their fortune in real estate, Hrycyk said.