A 94-year-old former guard at Auschwitz is being tried at a German court as an accessory to the murder of at least 170,000 people.

Key points: Nazi guard Reinhold Hanning goes on trial for accessory to 170,000 murders

Nazi guard Reinhold Hanning goes on trial for accessory to 170,000 murders Prosecutors say Nazi extermination depended on its guards

Prosecutors say Nazi extermination depended on its guards Former camp inmates will testify in the trial

Reinhold Hanning was 20 years old in 1942 when he started serving as a Waffen SS guard at the death camp in occupied Poland, where more than 1.1 million Jews were killed by the Nazis.

His trial is expected to be one of the last Nazi war crime trials.

There was a heavy police presence around the court in the western town of Detmold as Hanning walked in, wearing glasses and a dark brown tweed jacket and looking at the ground, for a session limited to just two hours due to his age.

Prosecutors said Hanning had joined the Waffen SS, the armed wing of the Nazi party, voluntarily at the age of 18 and fought in eastern Europe during the early stages of World War II before being transferred to Auschwitz in January 1942.

Accused by the prosecutor's office in Dortmund as well as by 38 joint plaintiffs from Hungary, Israel, Canada, Britain, the United States and Germany, Hanning will hear the testimony of former camp inmates in court.

One of them is Erna de Vries, who was deported to Auschwitz along with her mother in 1943, at the age of 23.

Considered a "Jewish crossbreed" as her father was Protestant, she was saved from the gas chamber and transferred to a labour camp.

"I survived, but to this day I don't know exactly how my mother was killed," she said ahead of the trial.

"The last thing she said to me was: 'You will survive and explain what happened to us.'

"I am not hateful but it somehow feels like justice to see this man, who was working there when my mother died, on trial."

Nazi murder machinery needed guards

Germany's Nazi war crimes office in Ludwigsburg found Hanning served as a guard at Auschwitz until at least June 1944.

He admitted to having been a guard, in a statement to the prosecution, but denied involvement in the mass killings, part of the Nazis' "Final Solution" for the extermination of Europe's Jews.

The entrance to Auschwitz carried the words "Arbeit macht frei" ("Work sets you free"). ( AFP: Joel Saget )

Investigators say he also served at Auschwitz's Birkenau sub-division, where about 90 per cent of more than 1.2 million killings in the camp were carried out in four gas chambers

Prosecutors maintain that the Nazis' machinery of murder hinged on people like Hanning guarding the prisoners, and they accuse him of expediting, or at least facilitating, the slaughter.

A precedent for such an approach was set in 2011 when death camp guard Ivan Demjanjuk was convicted of being an accessory to mass murder.

Last year, 94-year-old Oskar Groening, known as the "bookkeeper of Auschwitz", was sentenced to four years in prison for being an accessory to the murder of 300,000 people in Auschwitz.

Three other former death camp workers in their 90s — two men and one woman — are due to go on trial in the next few months.

Because of their age, their hearings, like Hanning's, will be restricted to two hours per day, assuming they are fit to face trial.

But Nazi hunter Efraim Zuroff, responsible for war crime investigations at the Simon Wiesenthal Center, said age should not be an obstacle to prosecution.

"When you think of these cases, don't think of frail, old, sick men and women," he said, "but of young people who devoted their energies to a system that implemented the Final Solution and aimed to obliterate the Jewish people."

Reuters