The Human Rights Watch (HRW) monitoring group says Iran has "recruited thousands of undocumented Afghans living there" to fight in Tehran-organized militias in Syria.
The January 29 report says Iran's Islamic Revolutionary Guards Corps (IRGC) has been offering Afghan refugees and migrants "incentives" to fight in Syria since at least late 2013.
Some of the Afghans that HRW spoke to said that "they were threatened with deportation back to Afghanistan" if they refused.
The report suggests that Iran's coercive tactics have prompted many Afghan males to flee Iran and head for Europe.
Iran officially claims the Afghans fighting in Syria are volunteers.
Some 3 million Afghans live in Iran, although only about 950,000 have legal refugee status there. "Their vulnerable legal position in Iran and the fear of deportation contribute to their decision" to fight in Syria, HRW writes.