Iraqis walk past a poster of Ahmed Hassan, an Iraqi Shiite fighter who was killed in Syria as he was protecting Sayida Zeinab shrine. Some Iraqi Shiites fighters have traveled to Syria claiming that their aim is to defend the shrine, which marks what is believed to be the grave of the granddaughter of the Prophet Mohammad. They say that they want to stop attacks by Sunni extremists on the shrine. (Karim Kadim/AP)

The Iraqi fighters in the video shoulder assault rifles and rocket-propelled grenades as they walk down a highway lined with cypress trees. Grinning, some hold up cellphones and camcorders to capture the moment — the aftermath of a victorious battle to secure the Aleppo airport from Syrian rebels who had attempted to take it.

“You are the sons of Iraq and the sons of Islam!” shouts one of their commanders. The men cheer.

Weeks later in Baghdad, Abu Sajad, the nom de guerre of an Iraqi militia commander who appears in the video, proudly displayed it as proof that Iraqi Shiites are playing a critical role supporting the regime of Syrian President Bashar al-Assad in what has become an increasingly sectarian and regional war. It was impossible to verify the location in the video or the circumstances.

Until recently, the involvement of Iraqi Shiites in Syria’s war was cloaked in secrecy here in Iraq, whose Shiite-led government has denied any role in the conflict. But recent interviews with militants, analysts, Arab government officials and residents of Shiite cities across Iraq reveal a trend that is growing increasingly open as Iraqi fighters come to view their participation as part of a regional struggle to defeat al-Qaeda and what they say is a broad effort by the region’s dominant Sunnis to wipe out Shiites.

At the center of the Shiite mobilization is Iran, which analysts and intelligence officials say is seeking to preserve its regional influence by funding and supplying an expanding Shiite network of armed support for the Syrian government, which is dominated by Assad’s Alawite sect, an offshoot of Shiite Islam. In addition to combatants from Iranian security forces and the Lebanese militia Hezbollah, pro-Assad proxy fighters include Iraqis drawn largely from militant groups known to be backed by Iran.

The role of Iraqi Shiite fighters in Syria raises questions about the possible complicity of the Iraqi government, which U.S. officials have recently criticized for allowing Iran to use Iraqi airspace for flights that allegedly transport weapons, troops and supplies to the Assad government. Iraqi officials say they have agreed to U.S. requests for inspections of the Iranian overflights. Eight recent random inspections have found “nothing illegal,” said Kareem Nouri, a Transportation Ministry spokesman.

“We support neither the opposition nor the regime in Syria, and we will not make Iraq a part of the fight in Syria,” he said.

But Iraqi officials have warned repeatedly that Assad’s fall would spell disaster for Iraq, and Iraqi Prime Minister Nouri al-Maliki told the Associated Press in February that a rebel victory in Syria would revive Iraq’s sectarian war. In an interview, Sami al-Askari, a Shiite lawmaker close to Maliki, said the government “turns a blind eye” to the flow of Shiite fighters to Syria, as it does in the case of Iraqi Sunnis who help Syrian rebels.

Analysts and Shiite militia leaders say it is unclear how many Iraqi Shiites have gone to fight in Syria, but Abu Sajad put the number at about 200 and said the ranks were growing quickly. He said Shiite fighters had been particularly motivated by an April statement by al-Qaeda leader Ayman al-Zawahiri, who glorified the Syrian opposition in what he depicted as its fight against Assad and Iran, and by the Syrian Islamist rebel group Jabhat al-Nusra’s recent pledge of fealty to al-Qaeda.

“Now it has become very common for people to say, ‘I’m going to Syria to fight,’ ” Abu Sajad said. “Why can Zawahiri say it publicly and we have to keep it a secret?”

Highly organized missions

In an interview in Baghdad, Abu Sajad and another Iraqi Shiite militia commander, Abu Aya, refused to say how they traveled to Syria or comment on Iran’s role in the process. But they said some of their operations helped tip the scale in favor of the Assad government, which has recently made gains against rebels.

Abu Sajad described his two-month mission this spring as extremely organized. He said that he took along 10 fighters, all highly skilled from years spent battling U.S. forces in Iraq, and that the Syrian army provided them with arms, vehicles and supplies.

“The Iraqi groups are only doing special missions,” he said. “We fight, and when we free a place . . . then the Syrian army comes in and sets up a base.”

The men said they were members of a Shiite militia but declined to say which one. Other Shiites who know them from Baghdad’s Sadr City neighborhood identified them as members of Asaib Ahl al-Haq, a group responsible for most attacks against U.S. forces in the final years of the Iraq war.

Residents and journalists in Baghdad and several Shiite cities in Iraq’s south said the group is leading a shadowy effort to recruit and dispatch fighters to Syria.

Publicly, militia leaders, government officials and Shiite clerics in Baghdad and Tehran say Iraqi Shiites are going to Syria exclusively to protect the Shiite Sayeda Zeinab shrine south of Damascus. Massoud Jazayeri, a spokesman for Iran’s armed forces general staff, told the Lebanese al-Manar news channel last week that “many measures have taken place” to form forces to protect Syria’s Shiite shrines.

But a growing number of news media reports about bodies that have been returned to Iraq from Syria and funerals for fighters slain there indicate that Iraqi Shiites are active in battles far beyond the Sayeda Zeinab district, where the level of combat is low, said Will Fulton, an Iran analyst at the American Enterprise Institute in Washington who co-authored a recent report on Iran’s strategy in Syria.

Abu Sajad and Abu Aya said there had been battles between Iraqi militants and anti-Assad rebels across Syria, including in Damascus, Aleppo, Homs and the strategic Qusair region along the border with Lebanon.

Iraqis, Hezbollah at the front

Residents of southern Iraqi Shiite cities said that fighters are mobilized in meetings with Shiite political parties and militias and that they often travel via Iran.

“Every day here, there are two or three funerals for martyrs killed in Syria,” said a journalist in Najaf who spoke on the condition of anonymity to avoid attracting the attention of the militias. He said about “90 percent” of the fighters had been mobilized by Asaib Ahl al-Haq and another ­Iranian-funded militia, Kataib Hezbollah.

Abu Sajad and Abu Aya said that in many instances, specialized paramilitary units of well-trained Iraqi Shiites and Lebanese Hezbollah fighters led offensives against rebel forces because Syrian army regiments were too afraid to do so.

The two men showed more than a dozen cellphone videos that they said Abu Sajad and his fighters shot during battles in Syria. Several other videos that purportedly show Iraqis fighting in Syria have surfaced on the Internet in the past two months.

One of Abu Sajad’s videos purports to show Iraqi fighters in green fatigues preparing for an assault on rebel forces in the Damascus suburb of Jobar.

“Look, that’s the Syrian army doing nothing because they’re scared,” Abu Sajad proclaimed, pointing to a cluster of men in half the frame. “And there’s me.”

Abu Sajad said his unit had helped deliver crushing defeats to the Syrian rebels, capture suspected spies and “liberate” Aleppo’s strategic airport from the threat of shelling.

By the end of his first mission in Jobar, Abu Sajad said, his unit — with the help of regular radio communications with Hezbollah — had pushed deep into a rebel-held territory and killed “a lot” of people.

Before proceeding with the offensive, he recalled, he told a Syrian army commander: “Now you will see what the Iraqis can do.”

When they were done, he said, they handed the area over to the Syrian army and moved on to the next mission.

Joby Warrick in Dubai, an Iraqi employee of The Washington Post in Baghdad, Sharaf al-Hourani in Cairo and Ahmed Ramadan in Beirut contributed to this report.