The trial of Sarah Shourd, Shane Bauer and Josh Fattal -- due to open on Sunday -- comes at a time when anti-American rhetoric is at fever pitch in Iran as it marks the anniversary of the 1979 Islamic revolution.

"I should have met with Shane and Josh to prepare the defence, but I was not allowed," lawyer Masoud Shafii told AFP. "I have been told, tomorrow I might see them one or two hours before the trial."

Shourd is expected to be tried in absentia as she was released on bail of around 500,000 dollars on September 14.

Shafii denied the charges against the three young Americans and said he would press for their "innocence" and immediate release.

Shourd, her fiance Bauer and fellow hiker Fattal say they innocently strayed into Iran from across the unmarked border with northern Iraq when they were detained on July 31, 2009 by Iranian authorities.

Iran has accused them of "spying and illegally entering the country."

"I have studied the case in full detail. The question of spying is irrelevant. There is just the question of illegal entry, which even if it has happened has been inadvertent as the border was unmarked," Shafii said.

"They are not at all at fault," he said, adding that illegal entry is punishable by a maximum three-year jail term which can also been commuted to a fine under the Iranian penal code.

He also said Shourd, who returned to the United States after her release on bail, was unlikely to appear in court.

"All the signs indicate that she will not be there. She has prepared a statement which has been delivered to me through the Swiss embassy," which represents US interests in Iran, he said.