Not long after a friend called from Damascus to tell him one of the holiest shrines in Shia Islam had been damaged by Syrian rebels, Baghdad student Ammar Sadiq was on the move.

Raging with a desire for vengeance, the 21-year-old set off for the border, a six-hour drive through Iraq's western deserts. He was one more jihadist on a road to war, a well-trodden path through lands that not long ago were used by jihadists coming the other way. When he got to Syria, however, he did not plan to join the Sunni insurgents now blazing through the north, but the equally vehement Shia groups defending the capital.

"It was like a thunderbolt hit me," said Sadiq. "My friend was telling me that wahhabis from Saudi and Afghanis were trying to destroy the [Shia] shrine of Sayyida Zeinab. I did not wait even to tell my parents. All I was thinking of is to go to Syria and protect the shrine, though I have not used a weapon in my life."

Sadiq was trying to join a group named Abu Fadl al-Abbas, which over the past 14 months has emerged as one of the most powerful in Syria.

Interviews with serving and former members of Abu Fadl al-Abbas suggest that upwards of 10,000 volunteers – all of them Shia Muslims, and many from outside Syria – have joined their ranks in the past year alone. The group's raison d'etre is to be custodian of Shia holy sites, especially Sayyida Zeinab, a golden-domed Damascus landmark, but its role has taken it to most corners of Syria's war. It is now a direct battlefield rival, both in numbers and power, for Jabhat al-Nusra, the jihadist group that takes a prominent role among opposition fighting groups.

Word of Abu Fadl al-Abbas has spread to Baghdad and elsewhere in the Shia diaspora. Many of its volunteers hail from Iraq's Shia heartland, where the group started some time last year with a fatwa delivered in Najaf by the renowned cleric Abu al-Qasim al-Ta'ai, who gave religious authority to the Shia going to fight in Syria.

The effect led to a surge of young Iraqis wanting to wage jihad and a groundswell of community support for a sectarian war in a neighbouring state, less than five years after similar bloodletting had ravaged Iraq.

Recruitment centres soon opened; militia leaders who had guided the rampage against the Sunni rebellion from 2004, first against the occupying American army, then against the ancient theocratic foe, were again mobilised. Cadres were called to arms, just as they were in 2006 when al-Qaida in Iraq succeeded – twice – in destroying another holy Shia mosque, the Imam al-Askari shrine in Samarra.

For Sadiq, however, joining Abu Fadl al-Abbas did not prove easy. First, Iraqi border guards advised him not to cross into Syria. They eventually let him pass after believing his story of trying to reach his family. He made it as far as Deir al-Zour, a city largely in control of rebels and the al-Qaida-aligned Jabhat al-Nusra, a group that no young Iraqi Shia wants to encounter without support.

Sadiq found the leaders in Damascus of Abu Fadl al-Abbas and soon learned that recruitment carried with it strict duties and obligations that he had not expected.

"The moment you join the brigade, you have to join the Syrian government army," he said. "You have to fight with President Bashar al-Assad before you fight for [the brigade]. The Syrian army will tell you that you have to know that you are protecting Syria, not only the shrine."

His quest wavering in the face of a very different role to the guard duty he had anticipated and relentless pressure from relatives back home, Sadiq gave up on his quest for jihad and returned to Baghdad.

Abu Fadl Al-Abbas has been more prominent in recent months than at any time since it started operating around in about March last year. Its increased role on the battlefields has come at the same time as Hezbollah has publicly stepped up its involvement, particularly in leading the attack on the border town of Qusair. Over the same period a weary Syrian army has had a boost in both morale and energy. A war that was starting to look unwinnable now looks to have an end point after all.

"There is no major fight anywhere, except the far north and east where Abu Fadl al-Abbas isn't deployed," said a Syrian businessman who has helped integrate Shias from outside Syria into the group. "Its influence is very important and growing."

The increased organisation of the group was evident in Baghdad, according to Sadiq. "The first step is to register with one of the Shia Islamic resistance offices, like Righteous League [Asaib al-Haq], Mukhtar Army or Iraqi Hezbullah."

Then comes a trip to a boot camp in Iran. "You have to enrol on a 45-day training course in Iran to be specialised in using a specific weapon like rocket launchers, Kalashnikov, sniper rifle or RPGs [rocket-propelled grenades]. After the course, you will be handed over to an Iranian middleman who will take you to Syria to join the brigade."

Murtadha Aqeel, 21, a college student from Baghdad, decided to join the jihadists in Syria at the end of 2011. He registered his name and was told that he had two choices, either to join the fighting near Sayyida Zeinab or in Darayya, south-east of Damascus, home to another Shia shrine, Sukayna, named after a daughter of Imam Hussain.

"If you go to Syria, you have one choice only, which is to die," Murthada said. "You stay for two or three months and come home for two months. Then you return."

Murthada trained with a Kalashnikov on the plains of southern Iraq; gruelling 12-hour days with a thousand other would-be jihadists. He said he was sent to Mashhad in Iran, then to Beirut, and on to Damascus by aeroplane.

"Once you get to the capital, there is a training centre near the shrine where all volunteers have to do a quick session of military training. Then they meet with Abu Ajeeb ([the commander of Abu Fadl al-Abbas] who asks all the volunteers to be careful and to go home safe," Murthada said.

"All of the volunteers come from abroad. We have everything to facilitate our fight. There are all kinds of weapons, no shortages at all. Three meals and hotels to host the fighters, mobiles and internet which are never cut."

In spite of the presence of the Sayyida Zeinab shrine, the battle to control the area, which is an essential approach to Damascus, has descended into a grinding but lethal stalemate.

"We face repeated attacks by the FSA [Free Syrian Army] all day, especially by mortars and artillery," Murthada said. "We were able to fortify the shrine ... but the mortars are giving us a hard time. The attacks get even more intense at night.

"Four of my colleagues were killed by snipers; one of them was Iraqi, another was Lebanese and the other two were Iranians. More than 35 others were wounded.

"There is no need for the Syrian army in Sayyida Zeinab. The brigade's fighters are protecting everything from the airport to the capital to Sweida [a Druze town near the Golan Heights], including residential areas, hospitals, government buildings, police stations, schools, mosques and hospitals."

Just over the barricades that now carve a jagged path through central Damascus and surround the gold-topped shrine, Syrian opposition fighters have been monitoring the prominence of the Shia group.

Almost all the rebel fighters, a mix of mainstream Syrians who want to replace Assad and jihadists whose battle has little to do with the country they are fighting in, rail against their enemy on the issue of Sayyida Zeinab, accusing the regime of using it as a pretext for inviting Shia fighters to join the conflict.

Abu Ahmed, an FSA commander operating near the Sayyida Zeinab shrine, said he and other Sunnis had no wish to damage it. Many in his ranks used to be local shopkeepers, whose livelihoods depended on the Shia tourist trade. He said the siege of the shrine began last July after a bomb killed four senior Syrian security figures in central Damascus. "The Shia went down to the streets with their arms and blocked all the roads and began to detain people," he said. "They killed a lot of our fighters. Then they began to gather around the shrine with members of Hezbollah, the [Iraqi] Mahdi Army and Syrian Shia.

"Since last July till today, we are fighting with them every day. We suggested a buffer zone around the shrine, but they refused. We are the biggest losers if the shrine is destroyed as we will lose our businesses," Abu Ahmed said.

A leader of Jabhat al-Nusra in Damascus, who called himself Abu Hafs, said: "These Shia fighters have been in Syria since the beginning of the revolution fighting with the regime. We know that Iran and Iraq are sending fighters to Syria – only now it has become public."

Jabhat al-Nusra, which includes large numbers of foreign fighters in its ranks, has made little effort to hide its hatred of the Shia branch of Islam and its willingness to attack shrines that are important to its followers.

Groups that fight under the banner of the Free Syria Army, however, are much less inclined to view the Shia as a theocratic foe, regarding them instead as a powerful backer of their main enemy, the regime.

"Now, they are in Qusair," said Abu Hafs. "They kill everyone they see on their way, even children. They slaughter them by knives. We are in a continuous fight with them in Damascus and Qusair.

"We worship God and they worship graves, but we also avoid attacking religious sites. A week ago, the Syrian army was hiding behind a church – we cancelled our attack in order not to destroy the church."

Abu Hafs's claim to be a protector of shrines is derided by Shia fighters. One of them, Jamal al-Ali, a member of Hezbollah who had volunteered to fight with Abu Fadl al-Abbas, said: "You have to know that the aim of these rebels is to destroy the Alawite state in Syria and to start that they have to destroy all the shrines. They are issuing endless calls for jihad against Hezbollah and Abu Fadl al-Abbas.

Back in Baghdad, Sadiq is preparing for a second bid at jihad. Hoping to make his next trip more successful than the last, he is waiting for a chaperone – a Lebanese woman based in the US – to take him to Beirut and finally back to Syria.

Whatever their motivations, the undeniable outcome is that both sides are now in open war across an ancient sectarian faultline in place since the schism in Islam emerged nearly 1,400 years ago.