Germany's culture minister Wednesday called on the nation's museums to ensure they are not housing any art looted by the Nazis.
The call from German Culture Minister Monika Gruetters' comes after the heirs of a Jewish art dealer and collector launched legal action over the southern state of Bavaria's refusal to turn over works of art allegedly seized and sold by the Nazis.
The heirs of Alfred Flechtheim, who fled Germany shortly after Hitler came to power in 1933, claimed in a New York court early in December that Bavaria had not met international commitments on the restitution of art taken during the Third Reich.
The culture minister called on museums to make use of a special process established in Germany for mediating in disputes about the ownership of works of art.
"Otherwise," she said, "it leaves the devastating impression that you want to keep something that is not rightfully yours."
Gruetters told dpa this "damages Germany's image in the eyes of the Jewish world community."
She went on to call on the authorities in the nation's 16 states to follow the lead of federal museums, which are morally obliged to provide information on the works of art that they hold.
Flechtheim's heirs have been negotiating with the Bavarian authorities for seven years for the return of 8 paintings by modern masters - Max Beckmann, Juan Gris and Paul Klee.
"What turns out to be a theft must be returned," said Gruetters. "There is no doubt about that at all."
After building up an art business in the 1920's promoting contemporary artists, Flechtheim fled to Paris in 1934 after facing Nazi harassment.
His galleries and business were later Aryanized by the Nazis and his private art collection was seized. He died in London in 1937.
Bavaria has rejected the Flechtheim family's claims that the paintings at the centre of the court case were looted art, saying that Flechtheim sold the works before the Nazis came to power.
But Flechtheim's heirs allege that the paintings were acquired by Hildebrand Gurlitt, one of a handful of dealers allowed to trade in modern art in Nazi Germany.
Gurlitt was also tasked with selling art stolen from Jews or works that had been confiscated after being denounced by Hitler's art functionaries.
Revelations three years ago that about 1,500 modern masterpieces pieced together by Gurlitt had been stored and hidden away in a Munich apartment by his son, Cornelius, caused a sensation in Germany and sent shockwaves across the global art world.
An expert team has already established that about 100 of the paintings from Gurlitt's controversial collection are likely to have been looted by the Nazis.
The priceless Gurlitt collection is now to be handed over to the Museum of Fine Arts in Bern.