Doctors have said the trial of accused Nazi guard John Demjanjuk can go ahead so long as there are no more than two 90-minute trial sessions a day. (Mark Duncan/Associated Press)

German prosecutors have ruled that accused Nazi guard John Demjanjuk is fit to stand trial for allegedly helping to kill 29,000 Jews in 1943.

Charges will be formally filed against Demjanjuk later this month, prosecutors in Munich said in a statement.

Doctors have determined the 89-year-old is fit to face trial on condition that court sessions do not exceed two 90-minute periods per day, the statement said.

Prosecutors allege Demjanjuk was a guard at the Sobibor camp in Nazi-occupied Poland in 1943, where they say he was complicit in the murders of 29,000 Jews.

The native Ukrainian has denied having any role in the killings. Demjanjuk says he was a Red Army soldier who spent the Second World War as a Nazi prisoner of war.

But Nazi-era documents obtained by U.S. justice authorities and shared with German prosecutors include a photo ID identifying Demjanjuk as a guard at the Sobibor death camp and say he was trained at an SS facility for Nazi guards at Trawniki, also in Poland.

The former autoworker was deported from his home in Ohio in May after losing an appeal to stay his deportation. Demjanjuk says he is gravely ill and suffers from severe spinal, hip and leg pain, a bone marrow disorder, kidney disease, anemia, kidney stones, arthritis, gout and spinal deterioration.

His son, John Demjanjuk Jr. said that German doctors have determined his father has about 16 months to live, due to his incurable leukemic bone marrow disease.

"With less than [two] years for my father to live, a career-seeking German prosecutor is hastily pressing forward, indicative of a 100 per cent politically motivated effort to blame Ukrainians and Europeans for the crimes of the Germans," Demjanjuk Jr. wrote.

"This has nothing to do with bringing anyone to justice or fitness for trial. My father will not live to fairly litigate the matter as [he] has successfully done before," he wrote.

Charges of accessory to murder carry a maximum sentence of up to 15 years in prison in Germany.

With files from The Associated Press