Scores of Iraqi Shi'ite militants are fighting in Syria, often alongside President Bashar al-Assad's troops, and pledging loyalty to Iran's supreme Shi'ite religious leader, according to militia fighters and politicians in Iraq, Reuters said.

Iraqi Shi'ite militia involvement in Syria's conflict exposes how rapidly the crisis has spiraled into a proxy war between Assad's main ally Shi'ite Iran and the Sunni Arab Gulf states supporting mostly Sunni rebels fighting the president.

The conflict has already drawn in a stream of Sunni Islamist fighters from across the region attracted to the rebel cause, while on the other side Syrian rebels accuse Lebanon's Shi'ite Hezbollah of supporting Assad's troops on the ground.

For Iraqi Shi'ites who follow Iran's Ayatollah Ali Khamenei, the uprising in Syria threatens Shi'ite influence and Iraqis fighting there say they see a duty to help Assad because of their loyalty to the Islamic Republic's highest authority.

Among them are defectors and former fighters from anti-U.S. Iraqi cleric Moqtada al-Sadr's Mehdi Army, the Iran-backed Badr group and Asaib al-Haq and Kata'ib Hezbollah, militias who once waged a bloody war on American troops, Shi'ite militants and Iraqi politicians say.

Shi'ite politicians say militants fighting in Syria have no official sanction from their militia leadership or from Iraq's Shi'ite-led government which is caught in a delicate balancing act between its ally Tehran, and Western and Arab powers calling for Assad to go.

Some of the Iraqi militants are former Mehdi Army fighters who took refuge in Syria after 2007 when their group was crushed by Iraqi forces. Others, loyal to Khamenei as a religious authority, crossed over recently, fighters and Iraqi politicians say.