WASHINGTON, USA - A friend of three Americans held by Iranian authorities after wandering over the Iraqi border into Iran called Monday for their release, in a letter to Iranian President Mahmoud Ahmadinejad.

In the letter, Shon Meckfessel called on Ahmadinejad to immediately free Shane Bauer, Sarah Shourd and Josh Fattal.

The three were detained in July after crossing into Iran while hiking in Iraqi Kurdistan. Meckfessel, who had traveled with his friends from Damascus for the trip, stayed in his hotel during the hike because he was feeling ill.

In his letter addressed to the Iranian leader, Meckfessel offered to submit a notorized statement to Iran's United Nations' mission, explaining the circumstances of their trip and vouching for his friends.

He described the detained Americans as longtime activists for humanitarian causes.

'Mr President, by continuing to deprive Shane, Sarah and Josh of their liberty, Iran is working against some of the very causes it supports,' Meckfessel wrote in the letter, published on the website of The Nation magazine.

'Each of these three has a long and public record of contesting injustice in the world and addressing some of the inequities between rich and poor which you have spoken about through their humanitarian work in their own country and overseas.'

Meckfessel wrote that Bauer is a fluent Arabic speaker who 'has focused on injustices in the Arab world, in Iraq and Palestine in particular,' and said Shourd helped rebuild homes after Hurricane Katrina, and helped poor people fight evictions from their homes in the United States.

He described Fattal as passionate about 'justice, environmental sustainability and intercultural exchange.'

'I hope that this letter will help the Iranian authorities understand the true character of my friends. They have now been in custody for almost 100 days, which I hope you agree is more than sufficient punishment for their mistake,' Meckfessel wrote.

A diplomat from Switzerland, which represents US interests in Iran, visited the three Americans in Iran's notorious Evin prison last week, according to their families, who said the trio were 'in good physical shape.'