(CNN) A former Nazi officer known as "the bookkeeper of Auschwitz" has been told he must serve out his four-year prison sentence, despite lodging an appeal for clemency.

Oskar Groening's plea for mercy was denied by German prosecutors in Lueneberg on Wednesday, one day after his appeal was made public.

Groening, 96, was sentenced in 2015 after being found guilty of being an accessory to the murder of 300,000 people at the Auschwitz death camp in Nazi-occupied Poland during World War II.

He was accused of counting the cash found in the belongings of new arrivals at the camp, where more than 1 million Jews died, and sending it to Nazi headquarters in Berlin.

During the trial, Groening admitted that he was "morally complicit" in the crimes but denied that he was legally guilty.

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