Christian Solidarity Worldwide said it fears Pastor Yousef Nadarkhani will be put to death despite reports that his sentence was annulled.
Nadarkhani was arrested in October 2009 while attempting to register his church and was found guilty of apostasy, or abandoning Islam, in September 2010. He was sentenced to death.
CSW said Iran's Supreme Court recently asked for a re-examination of his case to establish whether or not he had been a practicing Muslim adult before he converted to Christianity. However, the court ruled he wasn't a practicing Muslim, but is still guilty of apostasy because he has Muslim ancestry.
"We've had some reports that there has been a verbal announcement from the court in Iran that the sentence is annulled but we urge caution," a Christian Solidarity spokesman told Sky News. "It's been known that verbal announcements have been directly contradicted by later written statements. We are still calling for international pressure to be kept up."
Nardarkhani was born to Muslim parents but did not practice the faith. He converted to Christianity when he was 19.