A US-born journalist jailed in Iran on charges of spying for the United States has thanked those who helped win her release.

Speaking to reporters in Tehran on Tuesday, Roxana Saberi said she has no immediate plans for the future, but she expressed her joy at being free after four months in an Iranian prison.

"I am of course very happy to be free and to be with my parents again and I want to thank all the people all over the world ... who, whether they knew me or not, helped me and my family during this period," she said.

"At the moment I have no plan. I want to be with my family and relax."

Saberi was initially detained in Iran in January reportedly for buying alcohol, but was later charged with espionage.

She was sentenced last month to eight years in prison. But an Iranian court reduced her prison sentence on Monday to a two-year suspended sentence.

The ruling came after a court in the capital, Tehran, heard Saberi's appeal behind closed doors.

Confidential documents

Saberi's lawyers said on Tuesday that she was originally convicted in part because she accessed confidential Iranian government documents, which she obtained while working as a freelance journalist.

Abdolsamad Khorramshahi, one of her lawyers, told the Reuters news agency that Saberi had "accepted she had made a mistake and got access to documents she should not have. But there was no transfer of any classified information".

Saberi's father, Reza, said his daughter initially pleaded guilty to the charges under pressure, but later retracted her statements.

He said Saberi had not fully related her experiences in the prison but that she was not tortured.

"She is telling us little by little about the jail experience. The prison atmosphere is pressurised and not good for  anyone."

Saberi has reported for US National Public Radio, the BBC and Fox News, and has lived in Iran for the past six years.

On Monday, Barack Obama, the US president, welcomed what he called a "humanitarian gesture" by Iran.

The original sentence was handed down just weeks after Obama said that his administration would work towards better relations with Tehran after three decades with no official ties.

Washington has repeatedly denied that Saberi was involved in spying for the US.