The families of the three say they were hiking in Iraq and if they strayed across the border it was accidental [EPA]
The US government has said it will not pay prison bail for Sarah Shourd, a detained American who Iran says it will release on health grounds in exchange for $500,000, the US state department says.
"The United States government does not fund prisoner bail," Phillip Crowley, a spokesman for the department said on Monday, reiterating the standard US policy.
"We in the United States government would not be involved in that action, if that action were to be taken."
Shourd was arrested along with Americans Shane Bauer and Josh Fattal in July 2009 accused of spying and illegally entering the country from Iraq.
She was expected to have been freed on Saturday, when Mahmoud Ahmadinejad, the Iranian president, intervened as a gesture of Islamic compassion at the end of the holy month of Ramadan.
But the judiciary abruptly halted that planned release, citing legal technicalities.
Crowley said the prison bail was simply "one manner to resolve the situation," although without US government funds, calling the bail offer from Tehran "simply unclear."
Is bail legal?
Crowley also declined to comment on whether any potential transfer of funds to Iran, a country under sanctions by the United Nations and individual countries such as the United States, would be legal.
"There are transactions all the time between Iran and the rest of the world," he said. "Some of them violate sanctions, others don't."
While most financial transactions between Americans and Iran are banned under US sanctions, there are provisions for the treasury department to issue licenses for certain transfers.
A US official who requested anonymity said a bail payment initiated in the US could "probably" take place.
"But I don't know that the process has reached that point yet," the official said.
Earlier on Monday Shourd's family made an appeal, delivered by Swiss diplomats who handle US affairs in Iran, to lower the bail amount.
Masoud Shafiei, Shourd's lawyer, said there was no immediate word from Iranian authorities on the bid to drop or reduce her bail.
Shourd's mother says she has serious medical problems, including a breast lump and precancerous cervical cells.
The families of the three say they were innocent hikers in Iraq's scenic Kurdish region and if they did stray across the border into Iran, it was inadvertent.
"Right now our focus is on working as hard as we can to seek the release of all three of the hikers," Crowley said.