BERLIN—The U.S. and Israel are stepping up pressure on Germany to overhaul the way it handles Jewish heirs' claims for the return of art seized by the Nazis.
American and Israeli officials have begun lobbying Germany to make its overall policy on art restitution faster and more transparent—broadening the focus beyond the recent, high-profile case of 1,400 works found in a Munich apartment to include collections in German museums as well.
Many prominent people in the international art world have long said that Germany's procedures on art restitution are languid and opaque.
Claims on two paintings by Bernardo Bellotto, an 18th-century Italian artist, and on works by the German Expressionist Max Beckmann illustrate the problem. In both cases, lawyers for the heirs say they have been stymied by the refusal by German officials to submit to mediation.
The dispute has caused acute embarrassment in Germany, a country that prides itself on confronting its Nazi past—and its close ties to the U.S. and Israel. The relationship with Washington has already been frayed in recent months by German complaints about U.S. spying activities and U.S. criticism of Germany's economic model.
Germany's government office for restitution claims, known as the BADV, didn't return repeated calls and emails seeking comment. A spokesman for Chancellor Angela Merkel declined to comment.
The diplomatic pressure began after it emerged last month that German authorities had kept secret for nearly two years their discovery in a Munich apartment of some 1,400 artworks, many of which had been seized by the Nazis before and during World War II.
Now Germany is being criticized for failing to codify its international commitments on giving back Nazi-seized art, and for what critics charge is a lack of transparency from those tasked with processing restitution claims.
"No one can have a fair restitution-claims process without fair access to information," Stuart Eizenstat U.S. State Department's special adviser on Holocaust issues, said in an interview Monday.
He called on Germany to change its domestic regulations to uphold its international commitments on art restitution, a message that U.S. diplomats are also delivering to Berlin, according to U.S. officials.
An Israeli foreign ministry official said that his country is pressing Germany in separate talks to make its restitution proceedings more transparent. Israel is concerned that German museums could be "hiding" information about the provenance of valuable works, the official said.
Israel doesn't rule out asking for three-way talks with the U.S. and Germany if more transparency isn't achieved, the foreign-ministry official said.
In the latest incidents, the state-owned museums and Germany's government have declined to mediate the claims through the so-called Limbach Commission, an official German body tasked with handling Jewish families' claims to artworks held by German museums.
U.S. and Israeli officials say the German stance shows the need for changes to the country's laws.
They say the laws don't fulfill the Washington Principles on the return on Nazi-seized art, which the German government signed in 1998.
Those principles require rapid disclosure and return of or compensation for works that were wrongly taken from people persecuted by the Nazis.
Other countries, including Austria and Russia, have enshrined the Washington Principles in their national laws. Austria's commission on art restitution, set up in 1998, has processed about 300 cases involving art-restitution claims.
The Limbach Commission has processed seven cases since its creation in 2003. Claimants' groups complain that, unlike in other countries, the German commission doesn't explain the reasoning behind its rulings, and can't compel museums to accept its mediation.
The Limbach Commission's head, art historian Jutta Limbach, declined requests for an interview. Another member of the commission said Ms. Limbach had instructed members "never to talk to the outside."
Historians say the Bellotto paintings, estimated today to be worth about $20 million each, were sold under duress in 1938 from the collection of Jewish entrepreneur Max Emden to a Nazi dealer, who was scouting art for Hitler's planned "Führermuseum" in Linz, Austria.
German law presumes all art sales by Jews to the Nazis to have been under duress.
The BADV is declining to go before the Limbach Commission to discuss the case with Mr. Emden's heirs, saying the Washington Principles aren't binding and therefore can't be used to compel it to accept mediation.
"The Federal Republic's legally nonbinding self-commitment…is not aimed at subjecting the relevant regulations enacted by the [German] legislature to corrections," a BADV official wrote to Mel Urbach, a New York lawyer for the Emden family, in late November, in a letter seen by The Wall Street Journal.
The BADV declined to comment on the letter.
Mr. Eizenstat said in the interview that such thinking "would make a mockery" of the international norms that Germany has signed up to. "If the Washington Principles are irrelevant because of German law, why did they bother signing them?" Mr. Eizenstat asked.