United States and Iraqi officials believe Abu Bakr al-Baghdadi, the leader of Isis (Islamic State), has left operational commanders behind with diehard followers to fight the battle of Mosul, and is now hiding in the desert, focusing mainly on his own survival.

It is impossible to confirm the whereabouts of the "caliph", but US and Iraqi sources say an absence of official communication and the loss of territory in Mosul suggest he has abandoned the city, 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.

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Baghdadi 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".

At the height of its power two years ago, Isis ruled over millions of people in territory running from northern Syria through towns and villages along the Tigris and Euphrates river valleys to the outskirts of Baghdad in Iraq.