U.S. Secretary of State Hillary Clinton demanded on Thursday that Tehran immediately release an Iranian-American freelance journalist jailed in Iran for the past month.

Iran's judiciary said this week Roxana Saberi, a 31-year old who was born in the United States and reported for the British Broadcasting Corporation, National Public Radio and other media, was being held in Tehran's Evin prison.

"I am very concerned about this young woman," Clinton told a news conference at NATO headquarters in Brussels.

Clinton said Saberi had been held in an Iranian prison for the past month and that the United States, which does not have diplomatic relations with Iran, was seeking information on the journalist via its Swiss intermediary.

"We have pressed very hard. We will continue to do so. We believe there is only one outcome to this matter and that is for her to be released as soon as possible, to return home to her family in North Dakota as soon as possible."

Clinton thanked the Swiss for their help but said it was "frustrating" not being able to have any direct contact with the Iranians on the case.

The United States is reviewing its isolation policy on Iran, including whether to open up a low-level diplomatic office there.

In a first overture to Iran, Clinton announced on Thursday that the United States planned to invite Tehran to a conference on Afghanistan.

Iran said that a revolutionary court, which under Iran's legal system deals with state security issues as well as other areas of the law, had decided on Saberi's detention. She holds dual nationality.

An Iranian Foreign Ministry spokesman said on Monday Saberi had been working illegally in Iran after her press card was withdrawn about two years ago.

Saberi's father Reza said earlier this week from Fargo, North Dakota, that his daughter had been held in Iran since Jan. 31, ostensibly for buying a bottle of wine, and that there had been no information about her for more than two weeks. Buying alcohol is banned under Iran's Islamic law.

He confirmed her credentials as a correspondent had been revoked but said she had stayed in Tehran to pursue a master's degree and was doing research for a book about Iranian society.

NPR, ABC and BBC, all employers at one time of the reporter, demanded she be given access to a lawyer of her family's choosing and that an international organisation with responsibilities under the Geneva Conventions be allowed to see Saberi.

"If no charges are filed, we now urge her immediate release, and that she is given permission to return to her home country, the United States," the three news organisations said.

Uncertainty about Saberi's fate follows the detention of four Iranian-Americans who were visiting Iran in 2007, worsening relations between the long-time foes. The four were later released on bail and at least two of them left Iran.