Eighty-one-year old Thomas Selldorff, who fled Austria with his family before it was annexed by Nazi Germany in 1938, hopes an upcoming international conference will bolster efforts to return Nazi-looted art.

The Nazi's seized over 200 artworks owned by his grandfather, an avid art collector, as part of a policy of seizing Jewish property. So far, Mr Selldorff has been able to retrieve only two of the lost paintings.

"I want to be able to pass these things on to my family... I want them to have the link and an appreciation for some of the things my grandfather was involved with," said Mr Selldorff, who lives in the US and wants to exhibit the altar pieces by Austrian baroque artist Kremser Schmidt in a museum.

Some 65 years after World War II, experts say thousands of artworks confiscated by the Nazis, including masterpieces by art nouveau master Gustav Klimt and expressionist Egon Schiele, still need to be restituted to their rightful owners.

Government officials from around 49 countries, dozens of non-governmental groups and Jewish representatives will meet in Prague this week to review current practises. They are likely to sign a new agreement to step up restitution efforts.

Some participants hope the conference will lead to the creation of a central body responsible for publishing updates on countries' progress, which could prompt them to do more.

The task of restituting Nazi-looted works is an epic one. The Nazis formed a bureaucracy devoted to looting and they plundered a total of 650,000 art and religious objects from Jews and other victims, the Jewish Claims Conference estimates.

Artworks were auctioned off, handed over to national museums or top Nazi officials, or stashed away for a Fuehrer museum Adolf Hitler was planning to build in the Austrian town of Linz, where he spent a part of his youth.

"This is one way that Jews were made to pay for their own elimination," said art restitution expert Sophie Lillie.

At the end of World War II, some works were returned but many continued to circulate on the international art market or stayed put in museums, and it was only in the 1990s that there was a new burst of Holocaust restitution.

Austria is considered among the leaders of art restitution efforts, putting its larger neighbour Germany to shame. The Alpine Republic in 1998 passed a law governing art restitution and has since returned over 10,000 artworks. "There are a handful of countries that have achieved a lot," said Anne Webber, co-chair of the Commission for Looted Art in Europe, citing Austria, Holland and Britain.

Austria's Belvedere Gallery has had to restitute 10 paintings by Gustav Klimt, including two portraits of Adele Bloch-Bauer, which are among the artist's most famous works.

"Most countries have not even undertaken the work which was endorsed in Washington in 1998," said Ms Webber, referring to the non-binding Washington Principles agreed by 44 countries in 1998 as the framework for returning looted art.

Under the Washington Principles, countries agreed to identify stolen art, open up archives, publicise suspicious cases and "achieve a just and fair solution" for the Nazi-persecuted pre-war owners or their heirs.

Lawyers and experts say many countries have not enforced the principles and hope they will agree at the Prague conference on a transparent way to report on progress.

One of the main obstacles to art restitution is the difficulty in tracing the provenance and proving the ownership.

Gunnar Schnabel, a German lawyer and author of Nazi Looted Art said museums often "hold back any information they might have" about the murky war-time past of some of their works.

The unique nature of the Nazi regime also makes it difficult to legally define which art was looted or not.

"The Nazis were very inventive, and thought up lots of ways to expropriate someone of his belongings," said Christoph Bazil, head of the Austria's art restitution committee. For example, Jews sometimes were coerced to sell their art to Nazis and their sympathisers, or they had to sell paintings to fund day-to-day living because they were forced out of work or because they had to pay discriminatory taxes.

Some people argue that in cases where the original owners of the artworks received money for them, it was a legally valid transaction, while others say the discriminatory Nazi policies imposed on Jews mitigates that validity.

Even when claimants are successful at proving their ownership of an artwork, they have often been unable to retrieve the work of art due to rigid export bans on cultural patrimony.