A court order issued yesterday notes 89-year-old Johann Breyer has been hospitalised but doesn't cite his condition.
Breyer's attorney hasn't returned a call seeking comment. His lawyers have said he's in frail health.
Breyer has been in federal custody since his arrest last month at his longtime home in Philadelphia.
German authorities hope to try him on accessory-to-murder charges for his guard service at the Auschwitz death camp in 1944.
The retired toolmaker has denied taking part in the mass killing of Jews and others.