A man purported to be the reclusive leader of the militant Islamic State group Abu Bakr al-Baghdadi making what would have been his first public appearance, at a mosque in the center of Iraq's second city, Mosul, according to a video recording posted on the Internet on July 5, 2014, in this still image taken from video. Reuters

U.S. and Iraqi military officials have revealed they believe the leader of the Islamic State group, Abu Bakr al-Baghdadi, escaped the organization's embattled stronghold in Mosul as a major Iraqi offensive advanced on the jihadists' positions in their largest city, Reuters reported Wednesday.

The Iraqi offensive, backed by the U.S., Kurdish forces and Iran-backed Shiite Muslim militias, retook the eastern half of the city in January and began beating back ISIS militants from the western half last month. The city, once Iraq's second largest, was taken by ISIS in 2014. It was there that Baghdadi declared himself the leader of a global so-called "caliphate," compelling Muslims worldwide to join the ultraconservative Sunni Muslim movement that had taken large swathes of territory in Iraq and Syria.

As a coalition of opposing forces approached, however, monitored communications between the influential jihadist chief and his commanders dried up, according to U.S. and Iraqi intelligence.

While Baghdadi's location could not be confirmed, sources close to the anti-ISIS offensive reportedly said he often hid among sympathizers in remote desert outposts and distanced himself from combat operations. He took extensive measures to duck spies such as switching cars and couriers and rarely stayed in one place for too long.

Numerous reports have surfaced claiming to have captured, wounded or killed Baghdadi, none of which, including an Iraqi intelligence communication last month, have been confirmed.

A number of ISIS fighters and their families have abandoned Iraq and fled to neighboring Syria, where the group's de facto capital of Raqqa is located. As ISIS followers have escaped the group's collapsing eastern front, however, they have faced even more vicious resistance at their western borders. ISIS, which began as al-Qaeda in Iraq after the 2003 U.S. invasion and overthrow of President Saddam Hussein, took advantage of the chaos and became an influential force among the various groups attempting to overthrow Syrian President Bashar Assad since 2011. Since then, virtually all sides in the international conflict, including the Russia and Iran-backed Syrian army, Turkey-backed rebels, a U.S.-backed coalition and Kurdish forces have turned against ISIS.

Baghdadi, whose real name is Ibrahim al-Samarrai, was a hardline Sunni Muslim cleric and possibly an active militant when he was arrested by U.S. forces in 2004. He was later released and became the leader of ISIS' predecessor, the Islamic State of Iraq, previously al-Qaeda in Iraq, after the death of its former leader, Abu Omar al-Baghdadi, by a U.S. and Iraqi military rocket attack in 2010. He has rarely made public appearances, leading to rumors of his health, but his elusiveness has often been linked to the lengths he has taken to avoid targeting by enemy forces.