BAGHDAD - Militant Sunnis from Iraq have been going to Syria to fight against President Bashar Assad for months. Now Iraqi Shiites are joining the battle in increasing numbers, but on the government's side, transplanting Iraq's explosive sectarian conflict to a civil war that is increasingly fueled by religious rivalry.

Some Iraqi Shiites are traveling to Tehran first, where the Iranian government, Syria's chief regional ally, is flying them to Damascus. 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, 25, an Iraqi fighter, said 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.

Increasingly entangled

Lebanon, which has 100,000 Syrian refugees, was pushed to the brink when a Sunni intelligence chief was assassinated in a bombing many there blamed on Syria and its allies in Lebanon. Jordan, sheltering more than 180,000 refugees, has struggled to contain the violence on its border. 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, 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 in Baghdad, 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 Assad'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.

Battle over faith

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-Qaida, who have joined the fight against Assad much as they did in the last decade against the Shiite-led Iraqi government.

"Syria is now open to all fighters, and Al-Qaida is playing on the chords of sectarianism," 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 Assad as part of a "divine duty."

Abu Sajad said he joined the fight after the rebellion began, but recently returned to his home in Basra. "I can tell that things are going to be crazy in Syria," he said. "It's a sectarian war, and it's even worse than the one we had here, which was between the militias and the political parties. In Syria, all of the people are involved. You can feel the hatred between the Sunnis and the Alawites. They will do anything to get rid of each other."