"It's a story that is going to keep coming up," Clooney said at Berlin's Berlinale Film Festival, which marked the European launch of his new film The Monuments Men.
The movie tells the story of a special US military unit charged with tracking down and protecting art during the Second World War. "There is a lot of art that is still missing," he added.
In his comments, he also backed Greece's long-running campaign to retrieve antiquities looted by the Brits - notably the priceless Elgin Marbles currently on display in the British Museum.
"I think (Greece) has a good case to make," said Clooney.
"It would be a very fair and nice thing to happen." Clooney stars in The Monuments Men as Frank Stokes, the bespectacled art historian who led the mission to shield Europe's great works of art - not just from Hitler, but also from allied bombing and advancing Soviet forces.
The film's release in Europe is particularly timely.
It comes only months after the sensational find of more than 1400 modern classics in a Munich apartment owned by the son of a prominent Nazi art dealers.