The head of the World Jewish Congress Monday pressed the German government to publish an inventory of a recently discovered vast trove of art looted by the Nazis.
Ronald S. Lauder told Die Welt daily that time was of the essence with possible heirs now elderly and that "injustice" would continue as long as clarity was lacking.
"The German government must show these pictures," he told the newspaper, calling for the immediate compilation of an inventory to be published on the Internet.
"Valuable time has been wasted. Neither the possible claimants nor possible witnesses in the return process are getting younger," Lauder said, adding his voice to calls for a complete list to be published.
"Injustice will not be removed but continued so long as there is no clarity created about the owners."
And he warned that if nothing happened "we will raise the pressure".
Despite mounting calls, German prosecutors have refused to publish a full inventory of the works, citing a need for more time to fully catalogue them and for discretion in their probe.
The more than 1,400 artworks found last year stashed in the Munich flat of Cornelius Gurlitt include masterpieces by Picasso, Matisse and Renoir along with previously unknown works by modernist painters Marc Chagall and Otto Dix.
The case only came to light last week in a magazine report.
Gurlitt is the son of Hildebrand Gurlitt, a powerful Nazi-era art dealer and collector who acquired the paintings in the 1930s and 1940s.
Hildebrand Gurlitt had been tasked by the Nazis with selling works looted from Jewish collectors or seized as part of a crackdown on avant-garde, or what the Nazis termed "degenerate art" in exchange for hard currency.
Chancellor Angela Merkel's spokesman Steffen Seibert has called for openness in the case, while stressing that the prosecutors were responsible for the probe.