An Iranian court has sentenced a US journalist to eight years in prison on charges of spying for the United States, a defence lawyer has said.

Roxana Saberi, 31, was sentenced on Saturday following a trial that was conducted behind closed doors. 

"She has been sentenced to eight years ... I will appeal," Abdolsamad Khorramshahi, Saberi's lawyer, said.

Saberi, who has US and Iranian nationality, has been detained in the Evin prison in Tehran since January.

Saberi was sentenced despite calls for her release from Hillary Clinton, the US secretary of state, and diplomatic overtures to Iran by Barack Obama, the US president.

The US called the charges against Saberi "baseless and without foundation" and demanded her immediate release earlier this week.

Freelance journalist

Saberi, who had lived in Iran for six years, was working as a freelance journalist  when she was arrested in January.

She was initially accused of working "illegally" since her press credentials were revoked in 2006, but last week the charge was changed to that of spying for foreigners.

Saberi has reported for US-based National Public Radio (NPR), the BBC and Fox News.

Her parents appealed to Ayatollah Ali Khamenei, the Iranian supreme leader,
for her release last month, saying she was in a "dangerous" mental state.

It is the harshest sentence meted out by an Iranian court to a dual-national citizen on security charges.  

Several US-Iranians, including academics, have been detained in recent years on security charges but released after several months behind bars.