Oskar Groening arrives at the courtroom to listen to the verdict of his trial in Lueneburg, Germany, July 15, 2015.[Photo/Agencies]

The charges related to a period between May and July 1944 when hundreds of thousands of Jews from Hungary were brought to the Auschwitz-Birkenau complex in Nazi-occupied Poland. Most were immediately gassed to death.

Unusually for trials of former Nazi camp guards, Groening was been open about his past throughout the 17 days of hearings.

In their verdict, judges went beyond the 3-year sentence prosecutors had sought. Groening's defense team had argued for acquittal, contending that as far as the law is concerned he did not facilitate mass murder.

Kompisch said Groening deserved respect for having been open about what he did and having testified, but that given the enormity of the crime it would have been inappropriate to impose a lower sentence.

Both sides have a week to appeal, and both prosecutors and the defense said they would consider whether to do so. Defense lawyer Hans Holtermann said Groening remains free in the meantime, and given his age and the possible length of appeal proceedings it was uncertain whether he would actually go to prison.

Dozens of Auschwitz survivors and their relatives joined the trial as co-plaintiffs, as German law allows, though none were present for the verdict.

Lawyer Thomas Walther, who represents 51 co-plaintiffs, said that "it is an excellent verdict."

"For us, it not a big question of whether it is three, four, five, six years in prison - that was never a topic," he said. The survivors "are just happy that this trial has been carried through to the end and that there was a verdict."

Walther, a former prosecutor who investigated Nazi-era crimes, was instrumental in advancing the new legal theory that service in a death camp is enough to prosecute someone for accessory to murder. He said the verdict was a vindication that could lead to new prosecutions.

"It is really an unbelievable confirmation of our argument," he said.

Auschwitz survivor Leon Schwarzbaum, 94, who was not a co-plaintiff but came to see the verdict, said that Germany should have acted far earlier against Groening, but that the verdict was a "measure of justice."

"Unfortunately I can't forgive him for what he has done," Schwarzbaum said. "Maybe he took the ring from my mother's finger as she was forced off the train."