MOSUL, Iraq/WASHINGTON (Reuters) - U.S. and Iraqi officials believe the leader of Islamic State, 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, focussing mainly on his own survival.
It is impossible to confirm the whereabouts of the Islamic State "caliph", who declared himself the ruler of all Muslims from Mosul's Great Mosque after his forces swept through northern Iraq in 2014.
But U.S. 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, Islamic State 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.
U.S.-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 U.S.-led invasion of 2003, it has been slow going, in part because hundreds of thousands of civilians remain in harm's way.
The 100,000-strong Iraqi force fully captured the eastern half of Mosul in January, and commanders began an operation to cross the Tigris and take the western half last month. Progress has since been steady and the coalition says its victory is now inevitable, which would dismantle the caliphate in Iraq.
The intelligence sources point to a sharp drop in Islamic State postings on social media as evidence that Baghdadi and his circle have become increasingly isolated.
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."
Since then, sporadic Islamic State statements mention attacks carried out by suicide bombers at various locations in Iraq and Syria, but place no particular emphasis on Mosul, despite the city being the main centre of fighting.
Neither Baghdadi nor any of his close aides released any comment on the fall of the eastern part of the city in January.
The group's presence on Telegram, a social media network that had become its main platform for announcements and speeches, has tapered off. The coalition estimates that Islamic State activity on Twitter has fallen by 45 percent since 2014, with 360,000 of the group's Twitter accounts suspended so far and new ones usually shut down within two days.
In what is likely to be a major symbolic victory for the U.S.-backed Iraqi forces, they are now closing in on the area around Mosul's Great Mosque on the western bank of the Tigris, where Baghdadi proclaimed his caliphate.
More than half of the 6,000 jihadists left to defend the city have been killed, according to Hisham al-Hashimi, the author of the book "World of Daesh", who also advises the Iraqi government. Daesh is an Arabic acronym for Islamic State.
U.S. commanders sound upbeat and say the battle for the city is now in a late stage.
"The game is up," U.S. Air Force Brigadier General Matthew Isler told Reuters at the Qayyara West Airfield south of Mosul, adding that some of Islamic State's foreign fighters are trying to leave the city.
Those left behind to fight, mostly Iraqis, are putting up a "very hard fight" on the tactical level but they are no longer an integrated force, as coalition air strikes took out command and control centres, car bombs and weapon caches, he said.
"They have lost this fight and what you're seeing is a delaying action," he said.
Although the loss of Mosul would effectively end Islamic State's territorial rule in Iraq, U.S. and Iraqi officials are preparing for the group to go underground and fight an insurgency like the one that followed the U.S.-led invasion.
The "caliphate" as a state structure would end with the capture of Raqqa, its de facto capital in Syria, possibly later this year.
Raqqa is far smaller than Mosul, but mounting operations against Islamic State in Syria has been trickier than in Iraq, because the group's many Syrian enemies have mostly been pre-occupied fighting among themselves in a civil war since 2011.
Nevertheless, Islamic State has faced setbacks in Syria over the past year against three main foes: U.S.-backed Kurdish and Arab militias, the Russian-backed Syrian army, and mainly Sunni Muslim Syrian rebels backed by Turkey.
"The inevitability of their destruction just becomes really a matter of time," said Major General Rupert Jones, deputy commander for the U.S.-led anti-Islamic State coalition, adding that the group's leadership was now focussed on little more than survival.
The last official report about Baghdadi was from the Iraqi military on Feb. 13. Iraqi F-16s carried out a strike on a house where he was thought to be meeting other commanders, in western Iraq, near the Syrian border, it said.
Baghdadi, an Iraqi whose real name is Ibrahim al-Samarrai, is moving in a remote, mostly-desert stretch populated exclusively by Sunni Arab tribes north of the Euphrates river, according to Hashimi.
The area stretches from the town of Baaj, in northwestern Iraq, to the Syrian border town of Albu Kamal on the Euphrates.
"It's their historic region, they know the people there and the terrain; food, water and gasoline are easy to get, spies are easier to spot" than in crowded areas, he said.
The U.S. government has had a joint task force to track down Baghdadi which includes special operations forces, the CIA and other U.S. intelligence agencies as well as spy satellites of the National Geospatial Intelligence Agency.
But Baghdadi seems to have learnt the lessons from the 2011 capture and killing of Osama bin Ladin, and relies on multiple couriers and not just one, unlike the al Qaeda founder, say U.S. intelligence sources.
He also switches cars during trips, a lesson learnt from the 2011 drone strike that killed Anwar al-Awlaki, an al Qaeda figure in Yemen.
Baghdadi has not publicly appointed a successor, but Iyad al-Obaidi, also known as Fadel Haifa, a security officer under former dictator Saddam Hussein, is known to be the de facto deputy, according to Iraqi intelligence sources.
More than 40 leading members of the group have been killed in coalition air strikes, but the insurgency is likely to continue even if Mosul is captured and Baghdadi and his aides are killed, according to Iraqi security experts.
"There will be other commanders rising because the structure of the organisation remains," said Fadhil Abu Ragheef, an Iraqi security expert specialised in IS affairs.
Reporting by Isabel Coles in Mosul, John Walcott in Washington and Maher Chmaytelli in Baghdad; writing by Maher Chmaytelli; additional reporting by Mohammed el-Sherif in Cairo and Kylie MacLellan in London; Editing by Samia Nakhoul and Peter Graff