The UN High Commissioner for Human Rights urged Iran on Thursday to halt the execution of a woman convicted of murdering her husband when she was a child bride.
Navi Pillay also called on the Islamic republic to impose a moratorium on the death penalty in view of abolishing it altogether.
Razieh Ebrahimi, now 21, was forced to marry her husband at the age of 14 and gave birth just a year later, Mehr news agency cited her lawyer as saying.
At the age of 17, and after being subjected to domestic violence, she killed her husband and was later found guilty of murder, said the lawyer, who was not identified.
Ebrahimi had been due to hang for her crime in April, until the execution was postponed. In mid-June her lawyer urged the judicial authorities to scrap her sentence.
Pillay also appealed for clemency and expressed concern over the use of capital punishment in Iran.
"The imminent execution of Razieh Ebrahimi has once again brought into stark focus the unacceptable use of the death penalty against juvenile offenders in Iran," she said in a UN statement.
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"Regardless of the circumstances of the crime, the execution of juvenile offenders is clearly prohibited by international human rights law."
Pillay said imposing death sentences on people under 18 was "manifestly incompatible with Iran's international obligations under the International Covenant on Civil and Political Rights and the Convention on the Rights of the Child."
"I urge the Iranian authorities to halt the execution of Razieh and all other juvenile offenders."
The statement also highlighted the case of Jannat Mir, a 17-year-old Afghan boy who was hanged in April after being convicted in a drug-related case. It said five other Afghans were executed with him.
Pillay's office says that at least 160 people are reportedly on death row in Iran for crimes allegedly committed when they were minors.
More than 250 people are believed to have been executed in Iran since the start of 2014, while twice as many were known to have been executed last year, the UN says.
Pillay urged Iran "to immediately impose a moratorium on all executions, with a view to abolishing the death penalty."
Iran is the second biggest executioner in the world, after China, according to the United Nations.