Foreign militias have also reportedly been involved in the recent violent crackdown on large-scale protests in Iran, which is feared to have left thousands dead and injured.
According to US media citing security sources, members of the Lebanese Shia militia have been spotted leaving the country in the past 10 days.
The men were officially on pilgrimage, first to Iraq, home to the holy sites at the center of Shia Islam.
Nearly 5,000 militants from various armed groups have traveled to neighboring Iran in recent weeks to support state authorities, CNN reported on Thursday, citing Iraqi sources.
According to CNN, a European military source said there were more than 800 Shiites.
They are said to belong to groups under the command of Iraq’s so-called Popular Mobilization Units (PMU), an Iran-backed paramilitary network founded in 2014 to fight the Islamic State (IS).
It now plays a central political and military role in the country, partly as a parallel structure to the Iraqi state. The men, who are from Lebanon, are believed to be supporters of the Shia militia Hezbollah.
The demonstrations in Iran were sparked in late December by an economic crisis and soaring inflation, and later escalated into political protests against the country’s authoritarian leadership.
The Oslo-based Iranian Human Rights Organization (IHRNGO) said on Thursday that the number of demonstrators killed by security forces was 3,428, but the actual number is likely to be much higher.
These figures cannot be independently verified at this time.