Some Iraqi Shiites are traveling to Tehran first, where the Iranian government, Syria's chief regional ally, is flying them to Damascus, Syria's capital. Others take tour buses from the Shiite holy city of Najaf, Iraq, on the pretext of making a pilgrimage to an important Shiite shrine in Damascus that for months has been protected by armed Iraqis. While the buses do carry pilgrims, Iraqi Shiite leaders say, they are also ferrying weapons, supplies and fighters to aid the Syrian government.

"Dozens of Iraqis are joining us, and our brigade is growing day by day," Ahmad al-Hassani, a 25-year-old Iraqi fighter, said by telephone from Damascus. He said that he arrived there two months ago, taking a flight from Tehran.

The Iraqi Shiites are joining forces with Shiite fighters from Lebanon and Iran, driving Syria ever closer to becoming a regional sectarian battlefield.

Lebanon, which has 100,000 Syrian refugees, was pushed to the brink this month when a Sunni intelligence chief was assassinated in a bombing. Many Lebanese blamed the Syrian government and its allies for the attack. Jordan, sheltering more than 180,000 refugees, has struggled to contain the violence on its border, which claimed the life of a Jordanian soldier in a firefight with extremists last week. Turkey, with more than 100,000 refugees, has traded artillery fire with Syria since Syrian shelling killed five civilians near the border early this month.

Now Iraq, still haunted by its own sectarian carnage, has become increasingly entangled in the Syrian war. And Iran, which, like Iraq, is majority-Shiite, appears to be playing a critical role in mobilizing Iraqis.

According to interviews with Shiite leaders here, the Iraqi volunteers are receiving weapons and supplies from the Syrian and Iranian governments, and Iran has organized travel for Iraqis willing to fight in Syria on the government's side.

Iran has also pressed the Iraqis to organize committees to recruit young fighters. Such committees have recently been formed in Iraq's Shiite heartland in the south and in Diyala Province, a mixed province north of Baghdad.

Many Iraqi Shiites increasingly see the Syrian war — which pits the Sunni majority against a government dominated by Alawites, an offshoot of Shiite Islam — as a battle for the future of Shiite faith. This sectarian cast has been heightened by the influx of Sunni extremists aligned with Al Qaeda, who have joined the fight against the Syrian government much as they did in the last decade against the Shiite-led Iraqi government.

"Syria is now open to all fighters, and Al Qaeda is playing on the chords of sectarianism, which will spur reactions from the Shiites, as happened in Iraq," said Ihsan al-Shammari, an analyst and professor at Baghdad University's College of Political Science. "My biggest fear from the Syrian crisis is the repercussions for Iraq, where the ashes of sectarian violence still exist."

One young Iraqi, Ali Hatem, who was planning to travel to Tehran, then to Damascus, said he saw the call to fight for Mr. Assad as part of a "divine duty."

Abu Mohamed, an official in Babil Province with the Sadrist Trend, a political party aligned with the populist Shiite cleric Moktada al-Sadr, said he recently received an invitation from the Sadrists' leadership to a meeting in Najaf to discuss a pilgrimage to the shrine of Sayyida Zeinab, a holy Shiite site in Damascus.

"We knew that this is not the real purpose because the situation is not suitable for such a visit," he said. "When we went to Najaf, they told us it's a call for fighting in Syria against the Salafis," ultraconservative Sunni Muslims.

A senior Sadrist official and former member of Parliament, speaking on condition of anonymity, said that convoys of buses from Najaf, ostensibly for pilgrims, were carrying weapons and fighters to Damascus.

Iran, which has been accused of sending weapons and fighters to Syria, may have employed the same ruse. After the Syrian rebels detained 48 Iranians in Damascus in August, the Iranian government said they were pilgrims, and expressed outrage that they had been kidnapped by the rebels. According to American intelligence officials, at least some of the pilgrims were members of Iran's elite Islamic Revolutionary Guards Corps.

Yasir Ghazi reported from Baghdad and Tim Arango from Istanbul. Employees of The New York Times contributed reporting from Babil and Diyala Provinces in Iraq.