BERLIN—German museums are coming under growing international pressure to provide digital access to their full collections, in the wake of the discovery of a suspected plundered art trove in Munich that authorities kept secret for nearly two years. Under international norms adopted in Washington in 1998, German museums are obligated to go through their collections for works that may have been looted by the Nazis. But the museums have balked at going a step further and digitizing their collections to allow independent searches, citing budget restrictions and a lack of staff. That reluctance has for years been a source of tension within the art world, with critics alleging other motives. "They don't want to let people see what they have because they know if they put it online they'll get claims and possibly lose major paintings," Ronald Lauder, a billionaire art collector and president of the World Jewish Congress, said in an interview.

Ronald Lauder is the founder of New York City's Neue Gallerie, home to Gustav Klimt's "Portrait of Adele Bloch-Bauer", a work recovered after it was stolen by the Nazis.