"The death penalty cannot be imposed for crimes committed by persons below 18 years of age," insisted Cecile Pouilly, a spokeswoman for the Office of the High Commissioner for Human Rights.
The UN agency was "deeply dismayed" to learn of the reported execution of 21-year-old Ali Naderi last Wednesday for his alleged role in the murder of a woman when he was 17, she told reporters in Geneva.
She underscored that Iran was a party to several international treaties that "impose an absolute ban on the death sentence against persons below the age of 18 at the time when the offence was committed."
Last week's execution was the first of a juvenile offender in the country since September 2011, she said, acknowledging that it appeared authorities had "made efforts to prevent such cases."
But, she insisted, Tehran should move "to end the execution of juvenile offenders once and for all."
Pouilly also voiced concern for five other men — Mohammad Al Amouri, Sayed Jaber Shabain Alboshoka, Sayed Mokhtar Alboshoka, Hashem Shabain Amouri and Hadi Rashidi — who appeared to be at risk of imminent execution after Iran's supreme court upheld their death sentences.
"There are serious concerns about the fairness of their trials and allegations that they were subjected to torture," she cautioned.
Pouilly said more than 400 people were executed in Iran last year, and that most of them had been charged with drug-related offences. Pouilly said that under international law, such crimes are not considered serious enough to justify the death penalty.
The UN agency also condemned Iran for increasingly resorting to public executions, with more than 55 carried out in 2012 and several more so far this year, including the public hanging of two men in a Tehran park on Sunday.
"Executions in public add to the already cruel, inhuman and degrading nature of the death penalty and have a dehumanising effect on both the victim and those who witness the execution," Pouilly said.