An Iranian-American journalist branded a US spy has been jailed for eight years by Iran, her lawyer says.

Roxana Saberi, 31, was arrested in January and went on trial last week.

She worked briefly for the BBC three years ago, and has also worked for the American public radio network NPR and the TV network Fox News.

Saberi originally faced the less serious accusation of buying alcohol, then of working as a journalist without a valid press card.

Her trial was held behind closed doors in front of Iran's Revolutionary Court.

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

It is not known how long an appeals process might take.

The US had previously expressed its concern at Saberi's detention, dismissing allegations against her as "baseless".

US secretary of state Hillary Clinton has demanded her release.

However, public awareness of Saberi's situation is low in Iran, where local media do not seem to have reported her arrest or trial in any way.

A US-Iranian national, Saberi has spent six years in Iran studying and writing a book.

The daughter of an Iranian father and a Japanese mother, she was once crowned Miss North Dakota and was among the top 10 finalists in Miss America 1998.

She holds two master's degrees, from Northwestern University in the US and from Cambridge University in the UK, and is currently studying for a third.

Her arrest and trial has coincided with speculation about a thaw in US-Iranian relations, with US president Barack Obama offering a dialogue with Tehran on a range of issues.

In March Obama recorded a video message in which he offered a "new beginning" to Iran and its people.