BERN - A Swiss museum director preparing for a Naziera art collection's long-awaited public unveiling later this year said Friday that her goal remains finding heirs to any works that may have been looted from Jewish owners.

Bern Museum of Fine Arts head Nina Zimmer, who took ownership of 150 drawings, lithographs and paintings this week ahead of an exhibition slated to begin in November, said research shows none of these were stolen by National Socialists.

But questions linger over the provenance of some of the collection's pieces still in Germany, where a 2012 raid by authorities on a Munich apartment produced a sensation: 1,500 long-lost works by modern masters, including Pablo Picasso, Otto Dix and Henri Matisse.

"Every restitution is a victory for us," Zimmer said in an interview, while acknowledging such provenance sleuthing remains unpredictable. "I cannot make any promises."

Additionally, the Bundeskunsthalle in Bonn, Germany, is also planning to display items from the collection, which to date has produced only five works confirmed to have been stolen by the Nazis.

Four have been returned to heirs, so far, including a Matisse portrait, Sitting Woman, that belonged to Paris-based collector Paul Rosenberg.

Before its discovery five years ago, the massive trove was hidden for years in the German and Austrian homes of Cornelius Gurlitt.

His art-dealer father, Hildebrand, amassed it after being enlisted by the Nazis to sell so-called "degenerate" modern art they had seized from German museums.

Though original estimates for the collection's value topping $1 billion were likely exaggerated, experts said, the find is still spectacular.

"It is the most important cache of art from the Nazi era to be found in private hands since the immediate postwar period," said Jonathan Petropoulos, a Claremont McKenna College history professor in California.

When Cornelius Gurlitt died aged 81 in 2014, he named the Bern museum as benefactor. It accepted, on the condition works whose lineage was unclear must remain in Germany.

The Bern museum is now working with the German Lost Art Foundation, which tracks Nazi era art thefts, to unravel the collection's murky past, though not everyone is pleased with the progress.

Christopher Marinello, a lawyer who helped Rosenberg's heirs recover their lost Matisse in 2015, said the pace of research has been glacial - even after he provided German researchers with "full and complete provenance on a silver platter", he said.

"Internal and governmental bureaucracy in Germany is quite out of control," Marinello said in an email on Friday.

"There is an inherent lack of sympathy for the victims of Nazi looting."

The German Lost Art Foundation, which took over from a previous task force last year, contends it is making "positive strides" including digitizing documents and making them available via the country's Federal Archives.

The foundation is now scrutinizing 1,039 works from Gurlitt's collection, it said, of which 152 have produced some provenance evidence or claims from possible heirs that indicate they could be Nazi loot. Its work continues until December.

Reuters