Iran has secretly been recruiting Afghan Shia youth to battle the militant Islamic State (IS) group alongside Bashar al Assad's forces in Syria, an investigative report on The Guardian claims, — a fight that Afghanistan plays no official part in.

The recruits are usually poor, devout or social outcasts who are looking for money, acceptance or a sense of purpose that they cannot find at home, the report says.

A police officer and self-proclaimed 'travel agent' says the recruits are offered a residence permit in Iran and a $500 monthly salary to fight. "Most go to Syria for the money," he says. "Others go to defend the shrine."

The Sayyidah Zainab mosque in Damascus is a holy Shia site which has been a rallying point for members of the community who want to defend it from militants.

Iran, however, denies coercing or recruiting Afghans to fight in Syria, an embassy spokesman in Kabul said.

Iranian state media reports 20,000 Afghans are fighting in Syria, but does not acknowledge Iran's direct involvement. These fighters are often the first line of offensive action in Syria.

The Fatemiyoun Division, an Afghan Shia militia, is the second largest foreign military contingent fighting in support of the Syrian president after Hezbollah, a Lebanese militia.

The Guardian learnt that the division accepts fighters below 18 years of age subject to their parents' approval.

A new Iranian law grants citizenship to the family members of Afghans killed in the Syrian war, a move that is likely to attract more recruits and encourage fighters to take on more risky missions, Amir Toumaj, a researcher with the Foundation for Defence of Democracies believes.

Independent Iran analyst Ali Alfoneh says that providing war training to Afghanistan's Shia youth serves the interests of Tehran and the Revolutionary Guard, as these fighters can later go on to battle the Taliban or other groups on Afghan soil following the withdrawal of United States troops from their country.

Afghan MP Nazir Ahmadzai has accused Iran of fanning sectarian conflict by employing such tactics. "Iran's policy is to bring a division between Muslims," he claims.

Afghanistan's National Directorate of Security has made moves to clamp down on the recruitment of Afghan youth but is treading cautiously, believes security analyst Ali Muhammad Ali.

Because Afghanistan's relations with Pakistan are "unusually acrimonious", Ali said, Kabul cannot afford to antagonise Tehran as well.