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WASHINGTON — They have been called "the last prisoners of war" — the Renoir portraits, Monet landscapes, Cezanne still lifes and thousands of other artworks that remain scattered around the world.

Seized from thousands of public and private collections as the Nazis swept across Europe nearly six decades ago, the pieces, many of them masterworks, can be found on the walls of some of the world's most prestigious art museums — without any indication of their true provenance.

Long obscured in the shadow of the Holocaust and the Cold War, looted art has come into sharper focus recently as the search for Holocaust victims' assets has moved beyond dormant bank accounts and plundered gold to include the broader range of Jewish assets.

In the ongoing battle to secure moral and material restitution for Holocaust survivors, the issue of looted art carries a unique salience.

"A gold ingot or a gold brick is like any other gold brick," said Elan Steinberg, executive director of the World Jewish Congress. But a work of art is unique because "it tells a story of the family in whose parlor it hung.

"It tells the story of the transfer of ownership. It is almost a living object itself."

Beyond the difficulties in tracing an artwork's lineage, other issues are complicating Jewish efforts to reclaim the looted works:

*Convincing art museums to surrender or provide compensation for prized holdings.

*Determining what to do with heirless paintings.

*Weighing it all against the public stake in keeping the art on public display.

While the full scope of Nazi art plunder remains unknown, a document recently found in the U.S. National Archives offers some perspective.

In 1945, the director of the Metropolitan Museum of Art in New York estimated that the value of artwork plundered by the Nazis in Europe amounted to $2.5 billion in postwar prices — more than the value of all the art in the United States at that time.

In France alone, where many of the most notable art dealers maintained their collections, the Nazis seized 100,000 pieces of art, 39,000 of which were never recovered, according to France's own post-war accounting.

Another 16,000 artworks were not returned to their rightful owners. Those include some 2,000 paintings, drawings, sculptures and other works of art French museums selected for permanent display after the war.

Convincing European art museums — where most of the art resides — to relinquish their holdings has been difficult, given the differences in American and European laws regarding stolen property.

In America and Britain, the original owners of stolen goods maintain ownership rights, but in most of Europe, a buyer who purchases something in good faith is considered the rightful owner.

Indeed, European museums have taken great offense to charges they are displaying wartime loot, refusing to make such an admission. Jewish officials, meanwhile, have looked to U.S. museums to lead the effort by moral example.

But in a move that could portend further cooperation by European museums, Austria announced recently that it would return looted artworks now held by its museums.

"It is a wonderful irony that Austria will lead the way for the other European countries," said Steinberg, adding that "it was no accident" that the Austrian decision comes after the recent move by American museums.

Responding to growing concerns that U.S. museums may be displaying wartime plunder, the Association of Art Museum Directors, which includes the heads of the 170 largest art museums in North America, pledged last month to fully research the ownership history of their holdings.

In recent testimony before Congress, however, the directors of four of the most prestigious U.S. art museums said they were confident few of the tens of thousands of artworks stolen by the Nazis were in their collections.

Philippe de Montebello, director of the Metropolitan Museum of Art, said that even if the number of claims made against U.S. art museums in the last 50 years were tripled, "you would still be below 12 works of art and 12 claims."

But Jewish officials suggest that the number may increase dramatically because many of the paintings are heirless and more information has been made available through the declassification of documents in Eastern Europe in recent years.

Meanwhile, a task force set up by the museum directors has endorsed creating a mechanism such as mediation or arbitration to resolve ownership claims.

"We are committed to doing everything possible to ensure that our collections are untainted by the stigma of the Nazi past," Glenn Lowry, director of the Museum of Modern Art in New York, recently told the House of Representatives Banking Committee.

As a first step in resolving claims, two Jewish groups have teamed up to create a comprehensive database to identify the rightful owners of plundered art — a massive undertaking that will take at least six months.

The World Jewish Congress' Commission on Art Recovery and the B'nai B'rith Klutznick National Jewish Museum's Holocaust Art Restitution Project plan to post the database on the Internet.

While the database may help claimants obtain restitution, the more vexing question involves heirless artworks.

One idea is to sell the paintings at auction and use the proceeds for the benefit of needy Holocaust survivors, similar to the auction of looted art in the so-called Mauerbach collection in Austria two years ago. That sale raised $14.5 million for Austrian Holocaust survivors.

But while Jewish officials say compensation is a priority, they also recognize the importance of keeping the paintings in public view.

WJC officials have proposed establishing a museum of rescued art. It could take the form of a gallery housed in New York, Washington or Israel at a museum dedicated to the Holocaust or could function as a traveling museum or lending institution.

Steinberg said such a museum could achieve the dual obligation of keeping the paintings accessible, while also "respecting the memory and legacy of those from whom it was taken."

The idea is not likely to win many backers among art museum officials, some of whom have instead suggested placing special labels next to the looted art works, correcting their documentation and keeping them in their existing locations — a proposal ruled out by Jewish officials.