US and Iraqi officials believe the leader of Daesh, Abu Bakr Al-Baghdadi, has left operational commanders behind with diehard followers to fight the battle of Mosul, and is now hiding out in the desert, focusing mainly on his own survival.
It is impossible to confirm the whereabouts of Daesh's "caliph", who declared himself the ruler of all Muslims from Mosul's Grand Mosque after his forces swept through a third of Iraq in 2014.
But US and Iraqi intelligence sources say an absence of official communication from the group's leadership and the loss of territory in Mosul suggest he has abandoned the city, by far the largest population centre his group has ever held.
He has proved to be an elusive target, rarely using communication that can be monitored, and moving constantly, often multiple times in one 24-hour cycle, the sources say.
From their efforts to track him, they believe he hides mostly among sympathetic civilians in familiar desert villages, rather than with fighters in their barracks in urban areas where combat has been under way, the sources say.
At the height of its power two years ago, Daesh ruled over millions of people in territory running from northern and eastern Syria through towns and villages along the Tigris and Euphrates river valleys to the outskirts of Baghdad in Iraq.
US and Iran-backed Iraqi forces began an operation five months ago to recapture Mosul, a city at least four times the size of any other the group has held. The biggest battle in Iraq since the US-led invasion of 2003, it has been slow going due to a lack of preparedness and efficiency within the Iraqi military and supporting Shia jihadist formations.
The 100,000-strong Iraqi force fully captured the eastern half of Mosul in early February, and commanders began an operation to cross the Tigris and take the western half on 19 February.
The intelligence sources point to a sharp drop in Daesh postings on social media as evidence that Al-Baghdadi and his circle have become increasingly isolated. However, a steady stream of Daesh social media posts appears to contradict this, with regularly released combat statistic detailing Iraqi fatalities in the Mosul operation to be at over 7,000 men.
Al-Baghdadi himself has not released a recorded speech since early November, two weeks after the start of the Mosul battle, when he called on his followers to fight the "unbelievers" and "make their blood flow as rivers." However, lengthy silences are not unusual for the elusive self-declared caliph, as he has only made one public appearance, which was when he declared his caliphate in Mosul in 2014.
Mosul's Grand Mosque lies on the western bank of the Tigris River that bisects Mosul, and Iraqi units are closing in on it. Once it is recaptured, it will likely be a major symbolic victory for the US and Iran-backed Iraqi forces.
The US government has had a joint task force to track down Al-Baghdadi which includes special operations forces, the CIA and other US intelligence agencies as well as spy satellites of the National Geospatial Intelligence Agency.