Iran said on Monday a U.S. freelance journalist, whose father said was detained more than a month ago, had worked illegally in the Islamic Republic, an accusation likely to raise tensions between the two nations.

The father of Roxana Saberi said on Sunday 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.

Saberi, a 31-year-old Iranian-American born in the United States, has reported for the BBC, NPR and other media.

Asked about the case, Iranian Foreign Ministry spokesman Hassan Qashqavi said her press card was withdrawn about two years ago.

"This female reporter's press card was taken away ... and she should not have engaged in collecting news and information illegally," Qashqavi told a news conference.

"Her mission in collecting news and information from Iran ... has been completely illegal," he said, referring to the time since the press card was taken away.

He did not give details on her whereabouts, referring to Iran's judiciary for further comment.

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.

Saberi's case also comes as new U.S. President Barack Obama's administration has made clear it is interested in engaging with Iran to address suspicions it is seeking to obtain a nuclear weapon, as well as other matters. Tehran denies its nuclear program is anything but peaceful.

Reza Saberi confirmed on Sunday his daughter's credentials as a correspondent had been revoked but said she had stayed in Tehran to pursue a master's degree and was doing research for a book about Iranian society.

Iran and the United States have not had diplomatic ties for three decades and are now embroiled in a dispute over Tehran's nuclear program, which Washington believes is to develop a nuclear weapon but which Tehran says is for power generation.

In Washington, the State Department said it had asked Switzerland, which represents U.S. interests in Tehran, to seek information about Saberi from the Iranian authorities but had not yet received any response.

"Her status is unclear at the moment," State Department spokesman Gordon Duguid told reporters. "The State Department has been in touch with her family and we are working with them ... offering them what assistance we can."