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Thousands of Iraqi soldiers and Shiite militiamen sought to seal off ISIS fighters in Tikrit and nearby towns Tuesday, the second day of Iraq's biggest offensive yet against a stronghold of the radical Sunni Islamist militants, Reuters reported.

Iranian military commander Qassem Soleimani, who has helped coordinate Baghdad's counter-attacks against ISIS since it seized much of northern Iraq in June, was overseeing at least part of the operation, witnesses told Reuters.

His presence on the front line highlights neighboring Iran's influence over the Shiite fighters who have been key to containing the militants in Iraq.

In contrast the U.S.-led air coalition which has been attacking ISIS across Iraq and Syria has not yet played a role in Tikrit, the Pentagon said Monday, perhaps in part because of the high-level Iranian presence.

Iraqi military officials said security forces backed by the Shiite militia known as Hashid Shaabi (Popular Mobilization) units were advancing gradually, their progress slowed by roadside bombs and snipers.