The German Historical Museum said on Monday it was being sued to return a valuable poster collection seized by the Nazis from collector Hans Sachs to his son, who lives in the United States.

The museum, situated in the former East Berlin, said it was surprised at the claim by Peter Sachs as the West German authorities had paid his father "considerable" compensation for the collection in the 1960s.

Hans Sachs, a Jewish dentist, was the leading poster collector in Germany from the early 20th century until 1938, when he was sent to a concentration camp by the Nazis.

He was released after a few weeks, and fled with his family first to London and then to New York. Sachs, who is credited with raising the stature of commercial art, died in 1974.

A spokesman for the German Historical Museum said Sachs had accepted compensation in 1961 of 225,000 marks, equivalent to 600,000 euros or 718,000 dollars today, when the fate of the more than 12,000 posters in his collection was still unknown.

"Only later, in the mid-'60s, did he get a message to say that some of the works were in the museum in East Berlin," museum spokesman Rudolf Trabold told AFP.

Trabold said Hans Sachs was thrilled to hear that nearly 4,000 posters, including works by German artists Kaethe Kollwitz and Max Klinger, had been uncovered but made no attempt to lay claim to them.

"He said he was happy they were found, and he had been paid, so the matter was history."

Germany in 1998 agreed to return art confiscated by the Nazis to the families of the original owners, and has done so in numerous cases since.