By Chimaimba Banda and Sapa-AP

Nelson Mandela has expressed his confidence that the trial of 21 Iranians - 13 Jews and eight Muslims - accused of spying for the United States and Israel will be fair and just - but warned that the trial was a domestic Iranian matter and that interference from foreigners could jeopardise it.

The trial was due to get under way on Monday behind closed doors, with officials insisting it will be fair in the face of international misgivings.

"The hearing will take place in camera before the revolutionary court and in complete fairness," Iranian judiciary spokesperson Hossein-Ali Amiri said.

"The justice system does not involve itself in politics and acts impartially and in complete independence."

In a statement, Mandela said: "The families of the accused and the parliamentary representatives of the Jewish community are allowed to visit the accused. Because of security considerations, the presiding judge ruled that the trial should be held in camera.

"From all observations it would seem that the trial is fair and just," he added.