Previously unknown masterpieces by modernist painters are among a vast $1.3 billion trove of works including originals by Picasso, Matisse and Renoir, believed stolen by the Nazis and uncovered in a Munich flat.

Art historian and investigator Meike Hoffmann on Tuesday said that the new works by Marc Chagall and Otto Dix were found among thousands of other highly prized works.

The trove was found earlier this week in the flat of an eccentric elderly loner, Cornelius Gurlitt, the son of Hildebrand Gurlitt, a prominent Nazi-era art dealer who acquired the paintings in the 1930s and 1940s.

Hildebrand Gurlitt had been one of a handful of art experts tasked by the Nazis with selling valuable artworks stolen from Jewish collectors or seized among avant-garde works deemed to be "degenerate".

At a conference in Augsburg, Hoffmann, who is the chief expert aiding the investigation, said the Chagall painting, an allegorical scene dating from the mid-1920s, had a "particularly high art-historical value".

The Dix work is a rare self-portrait believed to have been painted in 1919, she added.

Rare masterpieces

Hoffmann showed slides of the paintings, which also include works by Pablo Picasso, Pierre-Auguste Renoir and Henri Matisse.

Augsburg chief prosecutor Reinhard Nemetz said 1,285 unframed and 121 framed paintings, sketches and prints were found in the rubbish-strewn flat, some dating back to the 16th century.

Focus magazine, which reported the story this week, said the the collection comprised 1,500 works worth an estimated $1.3 billion.

Nemetz declined to comment on the possible market value. Determining which works were looted from Jewish collectors by the Nazis or taken from them under duress for a fraction of their value would be a lengthy process, Hoffmann said.

Nemetz said that Gurlitt had co-operated with the authorities and that there was no warrant for his arrest.

His whereabouts are unknown. Gurlitt's official residence is in Austria and not Munich, where the art was found.

The Nazis plundered artworks in Germany and across Europe before and during World War Two

Thousands of stolen works have since been returned to their owners or their descendants, but many more have never resurfaced.

Jewish groups have called for an exhaustive search for the provenance of the paintings found in Munich, at least 200 of which were officially reported missing.

Source: Al Jazeera and agencies