The Iranian president has said that a US journalist convicted of spying should be allowed to offer a full defence during her appeal, a day after she was sentenced to eight years in prison by a court in Tehran.

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

Mahmoud Ahmadinejad sent a letter to Tehran's chief prosecutor instructing him to personally ensure that "suspects be given all their rights to defend themselves".

"Prepare for the court proceedings ... to observe and apply justice precisely,'' the state news agency Irna quoted him as saying on Sunday.

It was the first time Iran has found an American journalist guilty of espionage.

Saberi, who has US and Iranian nationality and has lived in Iran for six years, has been detained in the Evin prison in Tehran since she was arrested in January.

She has reported for the US-based National Public Radio, the BBC and Fox News.

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

Obama urges release

Barack Obama, the US president, said on Sunday that he was "gravely concerned" about Saberi's safety and was confident that she was not a spy.

"We are working to make sure that she is properly treated and to get more information about the disposition of her case, he said.

"She is an American citizen and I have complete confidence that she was not engaging in any sort of espionage."

He added that as "an Iranian-American who was interested in the country which her family came from", it was "appropriate for her to be treated as such and to be released".

Saberi's case has been an obstacle in US-Iran relations at a time when Obama is offering to start a dialogue between the countries.

Reza Saberi, Roxana's father, said that his daughter had been coerced into making incriminating statements about herself that she later retracted.

"She was deceived," he said on US National Public Radio.

"She is quite depressed about this matter and she wants to go on a hunger strike. And if she does, she is so frail it can be very dangerous to her health."

Human rights groups have repeatedly criticised Iran for arresting journalists and suppressing freedom of speech.

Tehran has arrested several Iranian-Americans in the past few years, accusing them of trying to overthrow the government through what it calls a "soft revolution".

Except for Saberi, they were never put on trial and were eventually released.