The smoke and din of the battle of Mosul have now come within a few miles of the landmark 12th-century mosque from which Abu Bakr al-Baghdadi declared the founding of a modern-day caliphate, ushering in a reign of terror in the ancient city on the plains of Nineveh.

Now, in the fourth week of a U.S.-backed alliance’s pincer-like move to retake the northern Iraqi city, the whereabouts of the Islamic State chieftain are unknown – though a rare audio recording posted online last week by the militant group and attributed to him urged followers to fight to the death.

Men are held by Iraqi national security agents, to be interrogated at a checkpoint, as oil fields burn south of Mosul, Iraq, last Saturday. Islamic State fighters launch counterattacks in the thin strip of territory Iraqi special forces have recaptured in eastern Mosul, highlighting the challenges ahead as the battle moves into more densely populated neighborhoods. Associated Press/Felipe Dana

A file image made from video posted on a militant website in 2014, purports to show the leader of the Islamic State group, Abu Bakr al-Baghdadi, delivering a sermon at a mosque in Iraq during his first public appearance. Associated Press/Militant video, File

Al-Baghdadi and his senior lieutenants have been the targets of an intensive manhunt by Iraqi and U.S. forces working from land and air, but experts disagree on how and where Islamic State’s top leadership might seek to regroup and recoup – if they are still present in the city, and if they manage to survive the battle of Mosul.

That’s a big “if” on both counts. Mosul’s inner precincts, which attacking forces have yet to penetrate, offered ample hideouts even before Islamic State fighters, during nearly 21/2 years of occupation, constructed an intricate warren of defenses above and below ground, including tunnels, trenches and secret passageways.

U.S. officials, who in late October disclosed the killing of 35 Islamic State commanders in Mosul over the last three months, say al-Baghdadi tops the target list.

“If we knew where he was, he would be killed at once,” Col. John Dorrian, the spokesman for the American-supported operation to retake the city, told reporters last week.

Al-Baghdadi, believed to be 45, is an Islamic scholar who is considered a prime architect of Islamic State’s apocalyptic ideology and its code of systematic and gruesome punishments of so-called infidels in areas under the group’s control.

He was also personally implicated by escaped captive Yazidi teenagers in the sexual torture of Kayla Mueller, the young American hostage who died last year in Islamic State-held Raqqah, the city that the caliphate calls its capital in Syria. It, too, is now under assault, with Kurdish-led Syrian rebels beginning to close in.

In the 31-minute audio address, Al-Baghdadi appeared to paint any setbacks – including the possible fall of Mosul to U.S.-backed Iraqi forces – as part of a larger strategy that would ultimately see Islamic State emerge triumphant. The group’s “great jihad,” he declared, bolsters “our conviction that all of this is a prelude to victory.”

In another signifier of the group’s long-term goals, the audio – whose authenticity has not been formally verified, but which several Western intelligence officials said they believe is probably genuine – calls on Islamic State’s followers outside Iraq and Syria to stage terrorist attacks against targets in the West, as well as against Saudi Arabia and Turkey.

Turkish forces have joined the fight against Islamic State in next-door Syria and have sought a role in the fight for Mosul.

Al-Baghdadi also directed particular threats at Shiite Muslims, who are considered apostates by the Sunni group and are now aligned in the coalition seeking to wrest the city from Islamic State. While militias drawn from the sect are barred from operations inside Mosul for fear of stoking tensions with the majority Sunni Muslim populace, a leading Shiite militia last week claimed it had gained control of a highway linking Mosul to Raqqah – which would have been a potential escape route for leaders trying to flee.

Whether they have already done so is a crucial question.

Fawaz Gerges, a professor at the London School of Economics who has written extensively about Islamic State and monitors jihadi communiques and activity, cited unspecified “tentative evidence” that the group’s top leadership remained entrenched in Mosul, once Iraq’s second-largest city and Islamic State’s greatest urban prize.

“Mosul’s urban core is very large, with a center 10 miles wide, and Iraq has always been the nerve center, the core” for the group, he said. “By contrast, Raqqah is a small city, and very exposed, and under tremendous American scrutiny.”

But with the Mosul offensive having been telegraphed for many months in advance, other close observers of the group believe senior cadres have probably slipped away and are directing operations from elsewhere. The secretive al-Baghdadi, who was once a prisoner of U.S. forces, is extremely mindful of operational security and communications that could allow coalition forces to track him.

Few photos of him exist; last week’s audio recording marked the first time in 10 months the Islamic State leader had been heard from. His greatest public exposure was the infamous June 2014 video, during the Muslim holy month of Ramadan, when the black-turbaned al-Baghdadi took to the pulpit at Mosul’s historic Grand Mosque to proclaim himself the emir, or leader, of the world’s Muslims.

Islamic State has shown itself to be nimble in its forward planning, and the group is probably already at work on ways to reassert itself even as its self-proclaimed caliphate slips away, likely reconstituting itself as a rural-based guerrilla insurgency that also calls on loyalists based in Europe and elsewhere to strike when and where they can in its name.

In the event of al-Baghdadi’s death, the group would probably convene a shura, or council, to choose his replacement, said Brookings Institution scholar William McCants, the author of a recently published book on the group, “The ISIS Apocalypse.”

“The internal factions are not well known, and it’s not clear how it will all shake out,” said McCants, adding that al-Baghdadi had come to power in what was essentially a rigged election.

“You’d have that kind of scheming this time around,” he said. “There is probably an heir apparent, but it’s not clear who it would be.”

Even if al-Baghdadi is eliminated, the group could probably go on without him, experts say, as it has after the previous losses of major figures.

“Together with the loss of territory and the decline in the number of foreign fighters coming to Iraq, it would certainly be a blow to the narrative if Baghdadi, for example, were to be killed,” said Seth Jones of the Rand Corp. But Islamic State’s predecessor organization demonstrated a remarkable degree of resiliency a decade ago, he said.

“They’ve rebounded from serious losses,” said Jones, who is the director of Rand’s International Security and Defense Policy Center.

Perhaps more important, the larger backdrop against which the group has thrived remains largely unchanged – including long-standing political grievances among Sunni Arabs in Iraq and Syria, deep mistrust of the institutions of the Iraqi government and vicious sectarian infighting, Jones and others said.

In any event, the immediate prognosis is grim for civilians in Mosul and elsewhere who are unfortunate enough to find themselves in the orbit of the group’s leadership, where they can be used as human shields or diversions.

“The question is how many thousands of civilians are going to be killed,” said analyst Gerges. “When you have a critical (Islamic State) mass, entrenched in a big city for 21/2 years, the capacity for killing is massive – I’m very, very alarmed.”

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