A US-Iranian reporter jailed by Tehran for espionage has gone on hunger strike and says she will not stop until she is released, her father said on Saturday.

Reza Saberi said his daughter Roxana, 31, had called from prison to say she was on her fifth day of refusing food. 'She's not eating anything. I'm very worried,' he told Reuters. Judging by her voice over the telephone, she seems to be weak,' he added.

The freelance journalist was sentenced to eight years in jail on April 18 on charges of spying for the United States, in a verdict that could complicate Washington's efforts towards reconciliation with Iran after three decades of mutual mistrust.

The United States has called the charges against Saberi baseless and demanded her immediate release. Her defence lawyer said he had appealed the sentence and expressed hope she would be acquitted, but he also voiced concern about her health following news of the hunger strike.

'I'm opposed to such an act since her health does not allow for that, as she is physically very weak,' Abdolsamad Khorramshahi told Reuters.

Last Tuesday, Reza Saberi said he believed she would starve herself to death if the verdict was upheld by an appeal court.

He said she shared a cell in Tehran's Evin jail with two other female prisoners and was 'not in a bad condition.' But she was physically frail and seemed 'desperate to get out of there.'

A citizen of both the United States and Iran, Saberi was arrested in late January for working in the Islamic Republic after her press credentials had expired. She was later charged with espionage.

US president Barack Obama has expressed deep concern for her safety and secretary of state Hillary Clinton has said releasing Saberi, who has worked for the BBC and US National Public Radio, would serve as a goodwill gesture.

Tehran, which does not recognise dual nationality, says Washington should respect the independence of Iran's judiciary.

But in a statement welcomed by her lawyer, president Mahmoud Ahmadinejad last Sunday called on Tehran's prosecutor to ensure that Saberi enjoys full legal rights to defend herself.

The judiciary chief has said her appeal must be dealt with 'in a careful, quick and fair way'. Lawyer Khorramshahi said the case would go to an appeal court in the next few days.

'I prepared a detailed bill last week and raised all legal issues in support of my client and the appeal bill was submitted today,' Khorramshahi said. 'I am hoping the circumstances for Roxana's release will be brought about.'

Reporters Without Borders, the Paris-based media rights group, has called Saberi's conviction 'unjust under the Iranian criminal code', saying it was a warning to foreign reporters working in Iran before its presidential election in June.

Amnesty International said she was a 'pawn to the ongoing political developments' between Iran and the United States.-Reuters