John Demjanjuk's trial in Germany was postponed Wednesday after doctors reported that the 89-year-old defendant was experiencing medical problems.

Presiding Judge Ralph Alt said the doctors at the prison hospital where Demjanjuk is being held reported he was suffering from dangerously low hemoglobin levels and needed treatment.

However, doctors thought the proceedings would be able to resume Thursday as scheduled, Alt said.

The retired U.S. autoworker is being tried on accusations he was the accessory to the murders of 27,900 people while allegedly serving as a guard at the Sobibor death camp. The Ukrainian-born Demjanjuk rejects the charges, saying he was never a guard at any Nazi camp.

Demjanjuk suffers from several medical problems but has been declared fit to face trial, so long as court sessions are limited to two 90-minute sessions per day.

Alt rejected motions from the defense for the trial to be ended, which argued, among other things, that Demjanjuk was no longer fit for the proceedings.

Even though Demjanjuk lies in a bed with his eyes closed throughout the court sessions, he has indicated he is listening and understanding what is going on, Alt said in his ruling.

Demjanjuk's family, however, said his condition has been getting worse as the trial has progressed.

SLIDESHOW: Alleged Nazi on Trial

"He has had five blood transfusions in Germany and they are becoming more frequent as his condition continues to worsen," his son, John Demjanjuk Jr. said in an e-mail to The AP. "We are hopeful he will endure long enough to be acquitted again."

Wednesday's session was the third so far to be abandoned due to health issues since Demjanjuk went on trial Nov. 30.

The trial in Germany comes after 30 years of legal action against Demjanjuk on three continents.

Demjanjuk had his U.S. citizenship revoked in 1981 after the Justice Department alleged he hid his past as the notorious Treblinka guard "Ivan the Terrible." He was extradited to Israel, where he was found guilty and sentenced to death in 1988, only to have the conviction overturned five years later as a case of mistaken identity.

Though the prosecution has said there are no known witnesses who can identify Demjanjuk as a guard, an 87-year-old Soviet war veteran who survived Sobibor said on Czech radio Wednesday that he remembered Demjanjuk from the camp.

The veteran, Alexej Weizen, said he recognized Demjanjuk from an old picture published in a Russian newspaper.

Defense attorney Guenther Maull expressed skepticism about the claim, however, noting that in a lengthy interview with Weizen in Germany's TAZ newspaper in 2008, Weizen mentioned many guards but never Demjanjuk.

Weizen also testified at a 1965 trial of Sobibor guards and Demjanjuk's name did not come up.

Nobody was available for comment at the special German prosecutors' office responsible for investigating Nazi-era crimes.