State Department spokesman P.J. Crowley said in a statement that at least 25 Baha'is were detained recently and as many as 60 were now imprisoned in Iran "solely on the basis of their religious beliefs."
A group of Baha'i leaders on trial for espionage were being denied access to their lawyers, Crowley said.
He added that Iranian authorities had detained more than a dozen Christians, "some of whom are being held in custody without substantiated charges."
"The United States is increasingly concerned about Iran's ongoing persecution of Baha'is and other religious minority communities," Crawley said.
On Thursday, the United States released its annual review of human rights conditions around the world, a report that portrayed Iran's record as worsening. Besides repressing religious groups, Iran, according to the report, continued to restrict freedom of expression and assembly and that 70 people had been killed and 4,000 more detained in a violent crackdown on protests.
Relations between the two countries have been tense, with the United States accusing Iran of developing nuclear weapons, a charge Tehran denies.
(Reporting by Richard Cowan; Editing by Peter Cooney)
