BAGHDAD — Militant Sunnis from Iraq have been going to Syria to fight against President Bashar al-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, 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.