Three young Americans detained in Iran over alleged illegal entry are to be charged with espionage, Iranian state news agency Irna says.
Shane Bauer, Sarah Shourd and Joshua Fattal have been held by Iranian authorities since the end of July.
The trio are thought to have crossed a poorly marked border by mistake while hiking in Iraq's Kurdish region.
The US urged Iran to free the group, while their relatives called the allegations against them "untrue".
Speaking in Berlin, US Secretary of State Hillary Clinton said that there was "no evidence to support any charge whatsoever".
A White House spokesman later called the group "innocent young people who should be released".
'Charges of spying'
Jon Leyne, BBC Tehran correspondent
The delay in charging the Americans suggests a prolonged debate behind the scenes in Iran over how to deal with them.
President Ahmadinejad hinted in September that their fate was linked with that of Iranians held by the US. No formal request for a prisoner exchange has ever been publicised.
There may also be some hardliners in Tehran who want to use the case to prevent any warming of relations.
Hillary Clinton has insisted that the trio are not guilty of anything. But her tone was relatively conciliatory. She requested their release, not demanded it, and called on the Iranian government to exercise "compassion".
Her moderate language suggests that, once again, Washington does not want the issue of Americans held in Iranian prisons to dominate relations between the two countries.
The three Americans were seized by Iranian border guards on 31 July near the town of Marivan. Their relatives say they accidentally strayed into Iran while hiking.
According to the state news agency, the move to charge them was announced by general prosecutor Abbas Jafari Dolatabadi.
"The three Americans arrested near the border of Iran and Iraq are facing charges of spying and the inquiry is continuing," Irna quoted him as saying.
The prosecutor said that an opinion on the case would be given "in the not too distant future".
In a statement, the Americans' families described the spying allegations as "entirely at odds with the people Shane, Sarah and Josh are and with anything that Iran can have learned about them since they were detained".
Swiss diplomats were allowed to meet the trio, who are in their 20s and 30s, in late September for the first time since their arrest.
The Swiss government represents US interests in Iran, with whom the US has no formal diplomatic relations.
In other comments, the general prosecutor said that the case of a Danish man arrested earlier this month was being investigated.
Niels Krogsgaard, 31, was detained on 4 November after anti-government protests that coincided with an official rally to mark 30 years since the storming of the US embassy.
"This person said he was a journalist though there were factors in his case which did not support it," Irna quoted the prosecutor as saying. "A journalist should have a permit. Inquiries are continuing."
Iran has tightened reporting rules since protests flared in the days following a disputed presidential election in June.
Opposition supporters say the elections were rigged to ensure the re-election of President Mahmoud Ahmadinejad.
At least 30 protesters have been killed in clashes since the elections. Thousands have been arrested, and some 200 opposition activists remain behind bars. Three have been sentenced to death.