Survivors of the Auschwitz death camp have told of their torturous experiences before a former guard goes on trial accused of 170,000 counts of accessory to murder.

Reinhold Hanning, 94, is accused of serving as an SS Unterscharfuehrer - or sergeant - between 1943 and 1944, a time when hundreds of thousands of Hungarian Jews were gassed to death at the camp.

He is one of four former Nazi guards being put on trial for war crimes this year, following an 11th hour push by German prosecutors. 

Survivor Leon Schwarzbaum, 94, the first witness to give evidence at the trial next week, still vividly remembers how the 'chimneys were spewing fire... and the smell of burning human flesh was so unbelievable that one could hardly bear it'.

Sat alongside other survivors, Schwarzbaum said it would be deeply unsettling to see Hanning in the courtroom on Thursday, but believed it was important to give the former SS man a chance to give a full account of the horrors that unfolded at Auschwitz.

He told a press conference: 'It's perhaps the last time for him to tell the truth. He has to speak the truth.'

Around 40 Auschwitz survivors or their relatives have joined the trial as co-plaintiffs, but not all will testify.

Hanning, who now lives in the small western city of Detmold, admitted to serving at the Auschwitz I part of the complex in Nazi occupied Poland but denied being at the Auschwitz II-Birkenau section where most of the Nazi's 1.1million victims were killed.

But prosecutor Andreas Brendel said the main camp's guards were also used as on call officers at Birkenau when trainloads of Jews were brought in.

He said: 'We believe that these auxiliaries were used in particular during the so-called Hungarian action in support of Birkenau.'

Hanning's case is one of 30 involving former Auschwitz guards being investigated by federal prosecutors from Germany's special Nazi war crimes office in Ludwigsburg. 

One of the two other cases likely to go on trial this year involves a 93-year-old woman charged with 260,000 counts of accessory to murder over allegations she served as a radio operator for an Auschwitz commandant in 1944.

Another 94-year-old man is charged with 1,276 counts of accessory to murder on allegations he served as an Auschwitz guard. 

Former SS officer Hubert Zafke, 95, is also set to go on trial at the end of February in Neubrandenburg, north of Berlin, on 3,681 counts of accessory to murder on accusations he served as a medic at an SS hospital in Auschwitz in 1944.

His attorney, Peter-Michael Diestel, said it was Germany's 'shame' that many higher-ranking perpetrators and Nazi war criminals were able to escape with minimal or no sentences in the years after the war.

He questioned whether prosecutors were simply trying 'to make up for mistakes of the past' with his client.

Diestel added: 'He was a medic for Wehrmacht [army] soldiers and SS men - for uniformed men - and had no part of the Holocaust - but the judicial argument of the Demjanjuk verdict says that if he didn't provide his service as a medic then Auschwitz wouldn't have functioned.

'What should a young man, even if he knew what was going on in Auschwitz, do to stop it?'

In all four cases, the health of the elderly defendants is expected to be a major factor on whether the trials can be concluded.

Hanning's trial will be limited to two hours per day because of his age. His attorney says his health will be checked again by an expert when the trial begins.

Jens Rommel, the head of the Nazi war crimes investigative office, says it is too early to talk about the final round of trials. 

There are a half-dozen open investigations right now with state prosecutors, and his office is looking into another seven suspects from both the Auschwitz and the Majdanek death camps.

'Year by year it's more difficult, but the state justice ministers last year decided that Ludwigsburg would keep working,' he says. 

'The state of North-Rhine Westphalia last summer talked about another 10 years as a timeframe.'

Auschwitz survivor Justin Sonder, who is scheduled to testify Friday, said it's never too late to pursue those responsible for running the camps.

'I'm often asked if there were SS men in Auschwitz who showed compassion,' the 90 year-old from Chemnitz said. 'No, absolutely not!'

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