Washington, which is locked in a dispute with Iran over Tehran's suspected pursuit of a nuclear bomb, said Akbar Ganji's wife and non-governmental groups reported his deteriorating health.
"His maltreatment at the hands of his captors has led him to the point where his health is at serious risk," US State Department spokesman Sean McCormack told reporters.
Mr McCormack said he did not know what health problem Ganji had developed nor whether it was life-threatening.
The United States, the European Union, UN Secretary-General Kofi Annan and human rights groups have called for Ganji's release.
He was jailed in 2000 after writing a series of articles linking senior officials to the murder of political dissidents.
Ganji also fell gravely ill in July, weakened by a hunger strike aimed at persuading authorities to free him.
Iran has rejected calls for his release and told foreign countries not to meddle.
The Iranian judiciary has faced domestic and international criticism for its poor human rights record while judiciary officials say they have been trying to reform.