Hossein Zebhi, assistant for judicial affairs to Iran's prosecutor-general, said in a statement carried by the IRNA news agency last week that a directive was issued commuting death sentences for offenders under 18 to life in jail.
The move was welcomed by Western rights groups which have criticised the Islamic Republic for sentencing youths to death for crimes committed while they were juveniles and carrying them out when they reach 18.
U.S.-based Human Rights Watch said it should save more than 130 offenders on death row.
But Zebhi appeared to take a different line in comments carried on Monday by the Etemad-e Melli daily, suggesting it was for the victim's family to decide the fate of the murderer.
"The principle of retribution … is not up to the government, rather it is up to the private plaintiff," Zebhi was quoted as saying. "Only if the next of kin give their consent can there be a reduction in the punishment."
In last week's report by the official IRNA news agency, Zebhi said offenders under the age of 18 would have their death sentences commuted to life in prison, regardless of the crime. He said the sentence could be reduced for good behaviour.
Human Rights Watch said in a statement on Friday a similar directive was issued in 2004 banning executions of people under 18 but that this did not stop judges issuing death sentences against juvenile offenders.
It said Iranian officials had justified some death sentences by arguing that executions in murder cases were not executions, but rather enforcement of the private right to retribution of the murder victim's family.
One Iranian lawyer said past directives issued by the judiciary had often been ignored because judges argued they were in conflict with Iran's Islamic law, sharia.
Iran regularly rejects accusations of rights abuses, saying it is implementing sharia. Tehran accuses Western countries of double standards.
Since January 2005, Iran has been responsible for 26 of the 32 known executions of juvenile offenders worldwide, Human Rights Watch said, adding that six juvenile offenders had been executed in 2008.
(Writing by Fredrik Dahl; Editing by Dominic Evans)
