State Department Spokeswoman Victoria Nuland said Nadarkhani has been in prison for 1,000 days.
"Pastor Nadarkhani still faces the threat of execution for simply following his faith, and we repeat our call for Iranian authorities to release him immediately," Nuland said Monday in a statement.
"The Iranian regime continues to deny and abuse the human rights of its citizens, in particular those of its many ethnic and religious minorities," she said.
CNN said Nadarkhani's next appearance in court is scheduled for Sept. 8.
The 34-year-old pastor, who is from the northern Iranian town of Rasht, converted to Christianity when he was 19. He was arrested in 2009 for protesting after his child was being forced to read from the Koran in school.
Nadarkhani was convicted of apostasy and sentenced him to death. Iran's Supreme Court says his sentence could be overturned if he recanted but Nadarkhani has refused.