Bearded and boisterous, the man wearing a chequered keffiya scarf sings a religious song as several Iranians in fatigues beat their chests feverishly.

The performance by Saeed Haddadian, a prominent Shia religious singer, took place among volunteers at the front line in Syria and was posted on the Internet in January, a sign of the increasingly high-profile political role being played by the "maddah" -- religious performers revered like pop stars among hardliners in Iran.

"In a country where music is outlawed and there is no outlet for young people to expend their energy, you have to have a replacement that is more official and without problems," said Mohammad Javad Akbarein, a former cleric and Shia scholar who studied in the holy city of Qom and now lives abroad.

"The maddah are copying from rap, rock and dance music," said Akbarein. "They perform their songs with the same intensity and excitement. Sometimes the songs are an exact copy of pop songs. They just take a love song and make it about religious love."

Most recently, they have played a prominent role in drumming up enthusiasm for the military venture in Syria, where a record 58 Iranians died last month helping protect the government of President Bashar al-Assad, according to the Washington Institute think tank.

"I went to Syria to show respect, admiration and deference to the fighters," Haddadian, who was photographed during his trip in a camouflage uniform strapped with ammo clips, said in an online video. "They do what we only speak about."

At least half a dozen well-known maddah have travelled to Syria and photos and videos of their trips have been posted online.

Within Iran, DVDs of maddah performances are sold outside of mosques. Maddah classes are offered in Tehran. Top maddah have been linked to the Islamic Revolutionary Guard Corps (IRGC), the most powerful military and economic force in the country, as well as the Basij, a popular militia overseen by the Guard.

Government organisations provide salaries, insurance, loans and pensions to some maddah.

Haddadian has performed for Supreme Leader Ayatollah Ali Khamenei and top Guard commander Qassem Soleimani. He has also been photographed with hardline former president Mahmoud Ahmadinejad, whose time in office from 2005-2013 saw the maddahs' status greatly enhanced.

"Ahmadinejad brought new life for the maddah," said Amir Farshad Ebrahimi, a former member of the hardline Ansar-e-Hezbollah Iranian militia who now works as a journalist outside Iran. "They are supported by the government but they are also used for specific purposes by the government."

PROPAGANDA

In their latest role serenading Syria volunteers, they have enjoyed the apparent sponsorship of the state. One of the first to go to the front, Hamid Reza Alimi, told an audience of young Basij members in a video posted online in 2013 that the Iranian embassy helped coordinate one of his trips.

Photos posted online show Alimi wearing a camouflage vest, pointing an AK-47 and posing next to a picture of Hassan Nasrallah, leader of Lebanon's Hezbollah Shia militia, which has fought openly in Syria alongside government forces.

"They use the maddah to gather forces to go fight in Syria," said Akbarein. "The maddah are very important for propaganda."

In particular, the maddah sing of the importance of defending holy places like the Seyeda Zeinab shrine near Damascus, burial place of a granddaughter of the Prophet Mohammad and one of the holiest shrines for Shias.

In recent months, the shrine was hit by multiple car and suicide bombs organised by Islamic State, the Sunni Muslim hardline group which considers all Shia to be heretics.

The appeal to defend the shrine is similar to the role that the maddah played during the Iran-Iraq war in the 1980s, when they called for the liberation of shrines in Iraq from Saddam Hussein's control. But the maddah are more organised now.

After Ahmadinejad's re-election in a disputed vote in 2009, some maddah performed at Basij rallies to encourage the volunteers to help suppress protests by Ahmadinejad's opponents. At least two prominent singers joined Basij volunteers confronting the protesters.

Since Ahmadinejad left power in 2013, government support and services for the maddah have continued, formalised through a "bonyad", a charitable trust largely funded by the government. Attempts to reach the singers mentioned in this story, the Guard media office or the charitable trust were unsuccessful.

Once the Syria conflict started, well-known maddah regularly performed at meetings of "defenders of the shrine" volunteers.

"The maddahs receive logistical support from the IRGC and Lebanese Hezbollah alike," Ali Alfoneh, an independent expert on the Guard based in Washington, wrote in an email. "Their deployment in Syria is a calculated part of the IRGC's military engagement in Syria."

They regularly sing at funerals of Iranian volunteers killed in battle, whose numbers have increased in recent months.

"They carry the coffin or perform," Ebrahimi said. "For the Islamic Republic, when a member of the Guard or Basij is killed in Syria that's the beginning of the process. They are brought back for a martyrdom procession. It becomes an outlet for the propaganda of the Islamic Republic."

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