The UN, EU and rights groups accused Iran of flouting its international obligations after Tehran's execution Wednesday of a young man convicted for a murder he allegedly committed as a minor.

Arman Abdolali, 25, was executed at dawn in Rajai Shahr prison near the Iranian capital over the killing of his girlfriend, despite international appeals to call off the execution.

His execution was in line with the "qesas" eye-for-an-eye style justice demanded by the victim's family, the Iranian judiciary's Mizan Online website said.

Amnesty International had appealed on October 11 for Iran to call off the execution of Abdolali, who was arrested in 2014 and convicted the next year of murdering his girlfriend, Ghazaleh Shakour.

The London-based rights group said he had been sentenced to death twice but the execution was stopped both times.

It described his sentencing in December 2015 as being the result of "a grossly unfair trial" by a court that "relied on torture-tainted 'confessions'".

Mahmood Amiry Moghaddam, head of Oslo-based NGO Iran Human Rights (IHR) which monitors executions in Iran, described Abdolali's execution as an "international crime".

"Besides being executed for an alleged offence committed at under 18 years of age which is a violation of international law, Arman was sentenced to death based on confessions extracted under torture, without a fair trial and due process," he said in a statement.

Moghaddam said the "repeated transfers for execution and then returning him without any explanation" constituted "psychological torture".