A 96-year-old man has been charged with complicity in the Nazis' murder of Jewish detainees while working as a guard at a concentration camp over 70 years ago.

The former SS guard, who has not been named, worked at the Madjanek concentration camp in occupied Poland between August 1943 and January 1944. Tens of thousands of Jews were killed at the camp during the Second World War.

He was a member of the SS Totenkopf unit, which regularly deployed guards to monitor and keep order at concentration camps.

Prosecutors in the German city of Frankfurt said the man "knew about the cruelty of the organised mass killings" at the camp and that the prisoners "facing their fate, innocently and defencelessly, were killed for inhumane reasons".

The man also allegedly supported "Operation Harvest Festival" in November 1943, when at least 17,000 Jews were shot in graves that the victims themselves were made to dig.

"He knowingly and willingly contributed to these insidious and cruel acts," prosecutors said. "By being part of a chain of guards and a tower guard, he made a contribution to [the operation] and knowingly and willingly supported the malicious and cruel acts."

Seventy years after senior Nazi officials went on trial in Nuremberg, Germany is racing against the clock to charge Nazi collaborators whose work supported but did not directly cause the murder of Jews, following decades of passivity.

Many cases fail to end up in court because the suspects are not considered fit for trial. In the past seven years only four suspects have been taken to court on charges of complicity of mass murder. A case against former Auschwitz doctor Hubert Zafke, 96, collapsed in September after he was deemed too ill to stand trial.