At a base near Tehran, Iranian forces are training Shiite militiamen from across the Arab world to do battle in Syria—showing the widening role of Iran's elite Revolutionary Guard Corps in Syria's bloody war.

The busloads of Shiite militiamen from Iraq, Syria and other Arab states have been arriving at the Iranian base in recent weeks, under cover of darkness, for instruction in urban warfare and the teachings of Iran's clerics, according to Iranian military figures and residents in the area. The fighters' mission: Fortify the Syrian regime of President Bashar al-Assad against Sunni rebels, the U.S. and Israel.

Iran's widening role in Syria has helped Mr. Assad climb back from near-defeat in less than a year. The role of Iran's training camp for Shiite fighters hasn't previously been disclosed.

The fighters "are told that the war in Syria is akin to [an] epic battle for Shiite Islam, and if they die they will be martyrs of the highest rank," says an Iranian military officer briefed on the training camp, which is 15 miles outside Tehran and called Amir Al-Momenin, or Commander of the Faithful.

The training of thousands of fighters is an outgrowth of Iran's decision last year to immerse itself in the Syrian civil war on behalf of its struggling ally, the Assad regime, in an effort to shift the balance of power in the Middle East. Syria's bloodshed is shaping into more than a civil war: It is now a proxy war among regional powers jockeying for influence in the wake of the Arab Spring revolutions.