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

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.

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