A reward of $200,000 US has been offered for information that could lead to the return of as many as 12 paintings by Marc Chagall, Diego Rivera and other masters that were stolen from a home in California's San Fernando Valley.

The stolen paintings were worth millions of dollars, Los Angeles police Det. Donald Hrycyk said on Tuesday.

It could be difficult for thieves to sell the works without proof of their authenticity.

The paintings disappeared on Aug. 23 from the home of an elderly couple. One of the residents is bedridden, and the other was asleep, so neither was aware of an intruder, police said.

The thief entered the house through a service door and took the paintings from the walls of two adjacent rooms.

Among the missing are pictures by Chagall, Rivera, Hans Hofmann, Chaim Soutine, Arshile Gorky, Emil Nolde, Lyonel Feininger and Kess van Dongen were stolen.

The stolen paintings were among a collection amassed by the couple, who are wealthy real estate investors, over the past 60 years or so.