California-based businessman and peace activist Ali Shakeri was released on bail on Monday after being held in Iran since May.

"Ali Shakeri was released on a one-billion-rial bail (107,000 dollars) yesterday," a judiciary source told AFP, adding that "he has to make a written request to the judge if he wants to leave the country."

The release came as Ahmadinejad started his third visit to the United States where he was to address the UN General Assembly amid controversy over his stance on Israel and Tehran's nuclear programme.

Shakeri was arrested along with US-Iranian academics Haleh Esfandiari and Kian Tajbakhsh, who faced similar accusations and were released on bail after spending more than 100 days in jail.

Shakeri, a board member of the Centre for Citizen Peacebuilding at the University of California, Irvine, was arrested during a private visit to Iran to visit his ill mother, his family said.

Iran had implicated academics Esfandiari and Tajbakhsh in an alleged American drive to overthrow the Islamic regime, but little was made public about Shakeri, who was also jailed in Tehran's Evin prison.

The arrests increased tension between Tehran and Washington at a time of growing concern about Iran's nuclear programme, which the United States claims is aimed at making an atomic weapon.

Esfandiari left Iran and returned to her job at the Woodrow Wilson Centre in Washington following her release on bail of three billion rials (320,000 dollars).

Officials said Tajbakhsh, who was released last week after posting a one-billion-rial bail, must make a written request in order to leave Iran.

The urban planner worked for the Open Society Institute of US billionaire George Soros, which is accused by the Islamic republic of seeking to foment a velvet revolution in Iran.

A fourth US-Iranian, journalist Parnaz Azima who works for the Persian arm of Radio Free Europe, Radio Liberty, was allowed to leave last week after being held for more than eight months.

Azima was not jailed but her passport was confiscated and she was accused of working for a "counter-revolutionary radio" station.

However, mystery still surrounds the fate of ex-FBI agent Robert Levinson, who Washington says went missing in March while on a private visit to Iran's Kish island. Tehran says it has no record of him entering the country.

On Monday Ahmadinejad appeared at Columbia University in New York where he was questioned and criticised over the country's treatment of academics, homosexuals and dissidents.

Leading Iranian dissident journalist Akbar Ganji urged the world on Monday to condemn what he said were human rights violations in Iran in a letter -- endorsed by 300 prominent academics from around the world -- to UN chief Ban Ki-moon.

But he strongly opposed the idea of a possible attack on Iran for its refusal to suspend controversial nuclear work, and said it made "things extremely difficult for Iranian human rights and pro-democracy activists."

Ganji's call came in the wake of warnings by Western countries, most recently France, about a possible military response to Tehran's alleged effort to develop nuclear weapons.

Iran has defied international calls to suspend its uranium enrichment programme, which can make nuclear fuel as well as the fissile core of atom bombs.

It denies it wants atomic weapons and insists that its nuclear programme is for peaceful civilian purposes only.