BAGHDAD Among the Iranian pilgrims, foreign executives and tourists in the departure lounge at Baghdad airport, a group of young Iraqis prepare to wage religious war in Syria - not for the rebels trying to topple President Bashar al-Assad but against them.
Dressed in jeans, their hair cropped short, the 12 men awaiting their flight are Iraqi Shi'ites, among hundreds heading for what they see as a struggle to defend fellow Syrian Shi'ites and their holy sites from the mainly Sunni Muslim rebels.
Syria is splintering the Middle East along a divide between the two main denominations of Islam, becoming a battlefield in a proxy war between Assad's main regional ally, Shi'ite Iran, and his Sunni enemies in Turkey and the Gulf Arab states.
The conflict has already drawn in streams of Sunni Islamist fighters on the rebel side, while Lebanon's Iranian-backed Hezbollah is openly fighting for Assad.
Now the flow of Iraqi militiamen across the border is also casting doubt on the Shi'ite-led Baghdad government's official position of neutrality in the Syrian civil war that has killed 90,000 people in more than two years.
For Ali, 20, fighting for the Abu al-Fadhl al-Abbas militia brigade meant joining his father in Syria to protect a Shi'ite shrine near Damascus from the Sunni rebels.
"It is my legitimate duty to go there and fight to defend Sayyida Zeinab Shrine," Ali told Reuters just before he left Baghdad last week. "Should we accept seeing Zeinab, the grand daughter of Prophet Mohammad, being captured again?"
As the Syrian war grinds into its third year, sectarian killings are increasing, while hardline Sunni clerics are declaring Jihad or holy war on the Shi'ites of Syria, Iraq and Lebanon. All this is inciting Shi'ite militants to fight back.
Reports abound that Sunni rebels have desecrated some of Syria's many Shi'ite shrines, including the Hojr Ibn Oday sanctuary, although these are often difficult to verify due to media restrictions in Syria and the general fog of war.
In the last few months, Iraqi Shi'ite militias have begun openly recognizing their formerly clandestine role in Syria, which has helped to double their recruitment, according to militia commanders.
However, this has also exposed schisms and infighting over the leadership of the Syrian and Iraqi militants who fight alongside Assad's troops.
Many Shi'ite fighters are young volunteers like Ali, but others are Iranian-trained militiamen who honed their skills against the U.S.-led forces which occupied Iraq until 2011.
Ali and other militants from around Iraq gathered recently at the Baghdad home of Abu Zeinab, a former senior leader from the Mehdi army militia, where they spent a few nights before travelling through the Baghdad airport to Syria.
Abu Zeinab said militant leaders took care of recruitment, equipment, flight booking and expenses, securing permits from the Syrian government and sometimes coordinating the different Shi'ite militant groups.
Militants say around 50 Iraqi Shi'ites fly to Damascus every week to fight, often alongside Assad's troops or to protect the Sayyida Zeinab shrine on the outskirts of Damascus, a particularly holy place for Shi'ites.
"The numbers of volunteers have significantly increased after Sunni rebel attacks which basically were targeting Shi'ites and the Shi'ite shrines in Syria," said Abu Zeinab. "We ask clerics whom we trust to register young men who they want to fight in Syria."
Iraq was dominated by its Sunni minority until the overthrow of Saddam Hussein in 2003, but now has a government led by members of the Shi'ite majority under Prime Minister Nuri al-Maliki. For them, Syria's upheaval is a nightmare; they believe a collapse of Assad's government will bring to power a hostile Sunni regime that will inflame Iraq's own Sunni-Shi'ite tensions.
Already sectarian attacks are resurging as al Qaeda's local wing and other Islamist Sunni insurgents regain ground in the western desert bordering Syria. Nearly 2,000 people have died in violence since April, with bombings targeting Shi'ite and Sunni mosques and neighborhoods as well as the security forces.
Iraq says it has a policy of non-interference in Syria, and keeps channels open with Assad's government and the opposition. But Western countries accuse Baghdad of turning a blind eye to support for Assad, such as allowing Iranian aircraft to use its airspace for flying military equipment to Syria.
Baghdad dismisses those charges and denies it is allowing Shi'ite militants to travel freely to Syria or giving them any logistical support.
"There has been an exaggeration of Iraqi brigades or units fighting in Syria. Really there has been a limited number of volunteers," Iraq's Foreign Minister Hoshiyar Zebari told Reuters in an interview. "These volunteers have gone there without any sanction or approval or support from the government or the Iraqi regime or the political leaders."
But Iraq's domestic politics are complex and some Shi'ite parties rely heavily on Iranian support, making them more sympathetic to Tehran's position on Syria.
Privately, Shi'ite politicians, officials and militant leaders acknowledge support is provided to Assad, and that means allowing Shi'ite fighters to flow into Syria.
"Shi'ite politicians believe the best way to keep Sunni extremist fighters out of Iraq is by keeping them busy in Syria," said a Maliki adviser, who talked in condition of anonymity because of the sensitivity of the matter. "All the Iraqi government has done so far, is to look the other way to the militant movements from Iraq to Syria," he said.
Militants usually fly in small groups of 10-15 from Baghdad and the Shi'ite holy city of Najaf, sometimes disguised as pilgrims. Their small bags may include uniforms, military equipment and sometimes pistols, militia fighters say.
Militia commanders say they have used their influence and the sympathy of Shi'ite officials in escorting fighters with their equipment through security checkpoints in Baghdad.
Most of those fighting in Syria are former members of the Mehdi army of anti-U.S. cleric Moqtada al-Sadr, or from the Badr Organization - the armed wing of ISCI political party - and the Asaib al-Haq and Kata'ib Hezbollah militias. Most are loyal to the supreme Iranian leader, Ayatollah Ali Khamenei, as a religious authority, fighters and Iraqi politicians say.
Some Mehdi army fighters took refuge in Syria when Iraqi and U.S. forces crushed their group in 2007. There they formed the Abu al-Fadhl al-Abbas brigade in coordination with the Syrian government and Khamenei's office in Damascus to defend the Sayyida Zeinab Shrine, militant commanders say.
Even experienced Iraqi militants had to join that brigade and fight under the command of Syrian Shabiha loyalists, who are mostly from Assad's own Alawite clan, an offshoot of Shi'ite Islam. This was a condition for being permitted and equipped by the Syrian Government, Iraqi militant leaders said.
Now the rules of engagement have changed, and splits have emerged among Syrian and Iraqi Shi'ite fighters. The Iraqi Mehdi army, Asaib al-Haq and Kata'ib Hezbollah have begun fighting under the command of the Lebanese Hezbollah, which helped Assad troops to recapture the strategic town of Qusair this month.
Military discipline imposed by leaders of Asaib al-Haq and Kata'ib Hezbollah on the Iraqis has irritated the Shabiha, some Iraqi fighters say, because the Syrians had tried to take advantage of the chaos to profit financially from the fighting.
Those disagreements erupted into a gunbattle near the shrine of Sayyida Zeinab few weeks ago between Asaib, Kata'ib and some Iraqi Mehdi Army fighters on one side and Abu Ajil, the Syrian commander of the Abu al-Fadhl al-Abbas brigade, and his local followers on the other. Two Iraqi fighters and three Syrian Shabiha died in the clash, militants in Baghdad said.
A reconciliation meeting was held under orders of Khamenei's office, but divisions fester and Iraqi combatants have formed a new brigade, refusing to fight under Syrian command.
"I am not taking a salary from the Syrian government, no one has a right to treat me as a mercenary Shabiha," said Abu Sajad, a former Mehdi Armi fighter, and one of the Shi'ite leaders who established the Abu al-Fadhl al-Abbas brigade. "I won't ever fight again by the side of those who killed my brothers."
(Editing by Patrick Markey and David Stamp)