The alarm was raised after a 19-year-old Iranian who had spent more than three years on death row was hanged at Adelabad prison in the southern-western city of Shiraz.

Behnam Zarei was put to death on Tuesday for killing a fellow teenager, identified only as Mehrdad, in a street fight in April 2005.

His execution came despite repeated international pleas for Tehran to honour treaties it has signed banning the execution of those under 18 at the time of their crime.

Zarei, who was 15 when arrested, was the second youth in days to be hanged for an offence committed as a child and the sixth such execution this year. He had told the court the killing had been an accident. He was hanged "without the knowledge of his lawyer and family", an Iranian newspaper reported.

"The situation of juvenile offenders facing execution in Iran has reached crisis levels," Amnesty International told The Scotsman. There are "at least 132 juvenile offenders known to be on death row in Iran".