The Iraqi security source said nearly 5,000 fighters from powerful Iraqi militias had entered Iran through two border crossings in southern Iraq, while the European source said hundreds of Shiite fighters crossed under the cover of religious pilgrimages.

The fighters were reported to be operating in several sensitive areas, including the western city of Hamedan, according to a European military assessment seen by CNN.

Iran International reported earlier this month that Iranian-backed Iraqi militias had begun recruiting and deploying fighters to assist Iranian forces in cracking down on protests.

That report said hundreds of Shiite militiamen from groups including Kataib Hezbollah, Harakat al-Nujaba, Kataib Sayyid al-Shuhada and the Badr Organization had been sent into Iran through multiple border crossings.

The fighters were transferred under the guise of pilgrimage trips and gathered at a base in Ahvaz before being dispatched to various regions, Iran International reported.