Human Rights Watch has accused Iran of recruiting Afghan immigrant children as young as 14 to fight in Syria.
In a report, the rights group said that the children fought in the Fatemiyoun division of Iran's Islamic Revolutionary Guards Corps (IRGC).
The division is made up of Afghan volunteers supported by Iran, fighting with government forces in Syria.
The HRW research was based on photographs of tombstones in Iranian cemeteries where combatants killed in Syria had been buried.
One of the eight graves showed a 14-year-old buried in the martyr's section of Tehran's Behesht-e-Zahra cemetery.
The HRW Middle East director Sarah Leah Whitson said, "Iran should immediately end the recruitment of child soldiers and bring back any Afghan children it has sent to fight in Syria."
There are an estimated 2.5 million Afghans living in Iran, according to Iran's interior ministry. Many do not have residency documents.
According to media reports, the Fatemiyoun division may number 14,000 fighters who have been trained and supported by Iran. Set up in 2013, there are no official statistics on the group.
One 29-year-old Afghan identified by HRW as "Ali" told the researchers in August that he joined the Fatemiyoun division after a recruiter told him he could renew his Iranian residency permit more easily if he joined up.
"They never asked me to show any documentation, but they wanted to make sure we were Afghan nationals,” Ali told Human Rights Watch. "We had to be above age 18 to be recruited, but they only asked for our age, not any documentation."
According to international law - the Rome Statute of the International Criminal Court – military recruits must be at least 18 and the training or deployment of soldiers younger than 15 is considered to be a war crime.