Iranian officials had said that Sarah Shourd, who was detained with her friends near Iran's border with Iraq, would be released on Saturday. But the IRNA state news agency quoted the deputy chief of communication for the Iranian president's office, Mohammed Hassan Salilhimaram, as saying that would not happen.
He said details of the decision would be announced later, but Tehran's chief prosecutor, Abbas Jafari Dolatabadi, blamed the fact that "judicial procedures have not been done," according to the semiofficial ILNA news agency.
It was the latest in a series of mixed messages from Tehran in a case that has deepened tensions between the U.S. and Iran, a relationship already strained over Washington's suspicions that Tehran is trying to manufacture nuclear weapons - something Iran denies.
Shourd and two friends, Shane Bauer and Josh Fattal, were arrested along the Iran-Iraq border in July 2009, and Tehran has accused them of illegally crossing the border and spying. Their families say they were hiking in Iraq's scenic north and that if they crossed the border, they did so unwittingly.
The U.S. State Department and relatives said they had no immediate information about the reports.
The announcement of the delay came hours after state media reported that President Mahmoud Ahmadinejad had personally intervened to secure Shourd's release as an act of clemency in part because of the "special viewpoint of the Islamic Republic of Iran on the dignity of women."
Patrick Clawson, deputy director at the Washington Institute for Near East Policy, said the last-minute quarrels over Shroud's release highlight the internal fissures in Iran's power structure between President Mahmoud Ahmadinejad and others such as the prosecutor who could see him overreaching his authority.
"There are all kinds of internal pressures," he said. "A case like this shows there are various factions at play."
A judicial official close to the prosecutor's office said that Dolatabadi believes the release is unacceptable because Shourd should first stand before the court and then the amnesty will be granted, but not before a court hearing.
The official spoke on condition of anonymity because of the sensitive nature of the issue.
Shourd's name was not among the official list of prisoners freed at the end of Ramadan, suggesting that prosecutors want the Americans to first face trial before any kind of pardon or clemency is considered.
Typically, inmates released during Ramadan have already been convicted.
In some recent cases of high-profile foreigners jailed and released in Iran, authorities have first conducted trials and issued sentences.
In May, a French academic, Clotilde Reiss, was freed after her 10-year sentence on espionage-related charges was commuted. American freelance journalist Roxana Saberi was convicted of spying before being released in May 2009.
Canadian-Iranian journalist Maziar Bahari of Newsweek was freed on $300,000 bail in October 2009 after nearly four months detention following the crackdown after the disputed presidential election. He was later sentenced in absentia to more than 13 years in prison and 50 lashes.
In 2007, Iran released 15 British sailors without a trial after being held for nearly two weeks for allegedly crossing into Iran along its river border with Iraq. Some were paraded on television to deliver confessions for trespassing.
The delay in Shourd's release is sure to further fuel concerns over her health, which her family has said is deteriorating.
The 31-year-old has been held in solitary confinement, and her mother has said she's been denied treatment for serious health problems, including a breast lump and precancerous cervical cells.
Shourd's planned release had provided a long-sought sign of hope to the Americans' families, who have been pleading with Iranian officials to free their children since their arrest.
Now, they are once again left wondering what is going to happen. "We don't know anything," said Samantha Topping, a New York publicist working with the families. She said the families knew only what they were hearing from media about a delayed release.
Associated Press writer Brian Murphy contributed to this report from Dubai, United Arab Emirates.