Iranian Supreme Leader Ayatollah Ali Khamenei has praised families of Afghan combatants killed in the fighting in Syria, according to a newly released video that could be aimed at boosting morale among troops deployed to support Tehran's regional ally, Syrian President Bashar al-Assad.

"I'm proud of you," Khamenei told the families in the March 27 meeting, video footage of which was first made public this week.

The release of the video on May 12 comes amid Tehran's mounting casualties in Syria, where 13 Iranian military personnel were killed last week in clashes with insurgents near Aleppo. It was Iran's heaviest single-day death toll in the conflict.

A few hundred Iranian and Afghan fighters are believed to have been killed in Syria during the five-year-old war, including several senior members of the Islamic Revolutionary Guards Corps (IRGC).

In June 2015, Iran's state IRNA news agency reported that about 400 Iranians and Afghans had been killed in Syria in the past four years.

About 150 more are believed to have been killed since then, according to reports by local media outlets that have been publishing their names and photos of their funerals.

Tehran says it has deployed only "military advisers" to boost Assad. In April, it said it has deployed a commando unit of its army to Syria to serve as "advisers."

Iranian media claim that Iranian and Afghan combatants who join the fight in Syria are "volunteers" who defend holy Shi'ite sites and are referred to as Defenders of the Shrine.

Afghans are being deployed as part of the Fatemiyoun Brigade, which reportedly consists of Afghan refugees, mainly Hazaras, living in Iran. Fatemiyoun's commander, Alireza Tavasoli, was killed in Syria in March 2015.

Human Rights Watch says thousands of them are undocumented Afghans recruited by the IRGC since November 2013, including some who, according to the rights group, have been coerced into joining the fight in Syria.

"Iran has not just offered Afghan refugees and migrants incentives to fight in Syria, but several said they were threatened with deportation back to Afghanistan unless they did," Peter Bouckaert, emergencies director at Human Rights Watch, said earlier this year.

In the March meeting, Khamenei told the families of some of the Afghans killed in Syria that "martyrs who die on this path are privileged."

"In fact your children have created a shield with their life to protect the holy shrines from these evil [forces]. Therefore their status is very important," he said.

Despite the rising death toll, Iranian authorities have suggested that Tehran will not accept any compromise on the fate of its ally, Assad, who has Russia's backing as well. The United States and its allies accuse Assad of indiscriminately bombing his own people and insist on his exit.

Iranian state media quoted Khamenei's adviser on international affairs, Ali Akbar Velayati, on May 8 as saying that Iran's red line is Assad remaining in power until the end of his term.