US-Iranian journalist Roxana Saberi, who has been detained in a notorious Tehran prison since January, has been charged with spying, deputy prosecutor Hassan Haddad said on Wednesday.
"Her case has been sent to the revolutionary court. She, without press credentials, was carrying out spying activities under the guise of being a reporter," Haddad was quoted as saying by the ISNA news agency.
"The evidence is mentioned in her case papers and she has accepted all the charges. She has been arrested under the laws of the Islamic Republic of Iran."
The decision to charge Saberi comes despite calls by US Secretary of State Hillary Clinton for her release and US President Barack Obama's diplomatic overtures to Iran.
On Wednesday, Clinton said Washington was very concerned after hearing of the Iranian charges against Saberi.
"We are deeply concerned by the news that we are hearing," Clinton said, adding the Swiss government had been asked to find out more given the lack of a US embassy in Tehran.
"I will, as will the rest of the (State) Department, continue to follow this very closely and we wish for her speedy release and return to her family," Clinton said.
Saberi, a dual US-Iranian national, was initially reported to have been detained for illegally buying alcohol.
In March, Iranian foreign ministry spokesman Hassan Ghashghavi said Saberi's press identity card was revoked in 2006 and since then she had been working "illegally" in the country.
Haddad said on Wednesday Saberi had entered Iran as an "Iranian citizen."
"She has an Iranian citizenship, passport and an Iranian national identity card. She has entered Iran as an Iranian citizen and if she has another citizenship, we are unaware of it and it has no effect on how we will proceed with her case," he said.
"There is no evidence that she has another citizenship and the investigation is still on."
Saberi's lawyer Abdolsamad Khoramshahi said he has still not seen the charges.
"I will go on Sunday to the court to seek authorisation for reading the charges, and to seek permission to meet her. I can't react to the charges till I have read them," he told AFP.
State Department spokesman Robert Wood said the United States was seeking more details about the case via the Swiss embassy in Tehran, which handles US interests in the absence of diplomatic relations.
"We have reached out to our Swiss protecting power to find out more information and try to confirm these charges," Wood told reporters.
US-born Saberi has reported for US-based National Public Radio (NPR), the BBC and Fox News, and had lived in Iran for six years.
Her parents, Reza and Akiko Saberi, arrived in Tehran on Sunday, and had a 20-minute meeting with her in Evin prison on Monday.
Her father said Roxana, 31, was surprised by their visit, and that she looked pale and weak but was in good spirits.
He said she also wanted to see her lawyer "to point out... that apparently some of the statements were made under pressure, under threat, you know. So that they were not valid."
Last month the parents appealed to Iranian supreme leader Ayatollah Ali Khamenei for her release, saying she was in a "dangerous" mental state.
Iran, which does not recognise dual nationality and has had no ties with the United States for three decades, has detained several Iranian-Americans, including academics, in recent years.
Clinton had delivered a letter to the Iranian delegation on the sidelines of an international conference on Afghanistan in The Hague on March 31, seeking Saberi's release and making appeals on behalf of two other US citizens.
Robert Levinson, a former FBI agent, vanished on the Gulf island of Kish two years ago, and student Esha Momeni has been prevented from leaving Iran despite being released from jail last year.
Momeni -- a graduate student at California State University -- was detained in Tehran on October 15 and released on bail in November, but has since been prevented from leaving the country.
She had travelled to Iran to carry out research on women's rights but was detained on national security charges.
Ghashghavi has denied receiving any letter from US officials asking about the three American citizens.