Iraqi troops have surrounded western Mosul and military leaders vow it's only a matter of time until they crush the last major stand of the Islamic State group in Iraq. But the militants are positioning themselves to defend the remains of their so-called "caliphate" in Syria and wage an insurgent campaign in Iraq.

The extremists are carrying out what looks like an organized, fighting withdrawal: a core of fighters is holding out in the city using hundreds of thousands of civilians as shields, tying down and bleeding the Iraqi military in urban combat.

Meanwhile, the Pentagon and Iraqi officials say the senior IS leadership has escaped to regroup in Syria and the deserts along the border to prepare for the future.

"They know they will lose Mosul, but they want this to be a hard fight," said Maj. Saif Ali, a commander in the Iraqi special forces on the front lines.

The civilian population is perhaps the main reason IS fighters have been able to hold out so long and turn Mosul into such a grueling battle. It took months for Iraqi forces to drive them out of eastern Mosul while trying to avoid high casualties among residents amid house-to-house battles. Now some 2,000 militants, by a coalition estimate, are holed up in western Mosul with 700,000 civilians. IS fighters are holding most of those civilians hostage as shields, while forcing some to flee as cover for their troops.

Mosul's fall will be the biggest blow yet to IS, largely breaking its hold over territory in Iraq and ending its rule over half the "caliphate," which at its height stretched from northern Syria through western Iraq. The largest city in IS territory, Mosul provided the group significant financing from taxing the population, factories to make weapons and space to gather freely.

But the Islamic State group's durable organization ensures it can fall back to the next fight.

THE BATTLEFIELD

Last weekend, Iraqi forces completely encircled western Mosul by capturing the last road into the enclave of about 40 square kilometers (15 square miles), comprising some of the city's most densely built districts.

"Any of the fighters who are left in Mosul, they're going to die there because they are trapped," Brett McGurk, the special presidential envoy for U.S.-led coalition against IS, said Sunday.

In the month since the assault on the west began, troops have retaken the city's airport, a sprawling military complex, the main government compound and a ribbon of neighborhoods on the southwest side of Mosul. The offensive is being waged from three directions with two divisions of special forces and a force of federal police advancing along the Tigris River, which divides the city into its western and eastern half.

Use of artillery and airpower has been dramatically stepped up, mostly by the Iraqi air force. "They are causing the real destruction," said federal police Cpl. Abbas Takleef, whose unit retook the government complex last week.

Iraqi officers have frequently expressed impatience with extensive vetting of airstrikes required by the U.S.-led coalition to avoid civilian casualties. Recently, the coalition enabled more of its officers on the scene to approve strikes, speeding up procedures.

But more intense bombardment could translate into deaths among residents. Airwars, an independent group that tracks casualties from the campaign, said several hundred civilians have been killed in March alone.

THE CIVILIANS

Residents trapped in western Mosul face dwindling supplies of food and fuel. Limited provisions enter through smuggling routes still in use despite the siege, according to a senior humanitarian official who spoke on condition of anonymity in line with regulations.

But prices have skyrocketed: the price of a kilogram (2.2 pounds) of sugar has leaped from $1 to more than $20.

Bassam and his wife Asma, residents of a western neighborhood just recaptured by Iraqi forces, said they survived because they began stockpiling provisions back in October, before the Mosul assault began. Shops completely ran out of food more than two months ago, they said.

The couple asked to only be identified by their first names, fearing for the safety of relatives still under IS rule.

Around 50,000 civilians have fled in the past four weeks, according to the United Nations. Escape is incredibly dangerous: IS has threatened to kill anyone caught trying to get out, and residents have to cross dangerous front lines to reach safety.

But at times, the militants allow large groups to leave, giving cover for their own fighters to move as well.

Ali, the special forces major, said that one night more than 5,000 civilians crossed out through front lines near his position. Soon after, militants who had slipped into government-held territory struck his forces from the rear, hitting a house and nearby school being used by the Iraqi troops. A sniper with a night scope killed a young soldier on the roof of the house, and a rocket-propelled grenade wounded a soldier at the school.

The attacks rattled the soldiers as they tried to regroup for the next push, said Lt. Col. Nour Sabah, who was stationed at the school.

"They are trying to exhaust us," he said.

WHAT'S NEXT FOR THE ISLAMIC STATE GROUP

The well-organized IS counter-attacks point to how the militants are maintaining command and control even as their grip on Mosul falls apart.

The Islamic State group's top leader Abu Bakr al-Baghdadi and his senior lieutenants escaped Mosul even before the assault on the city began in mid-October, Iraqi and coalition officials believe. They likely went to Raqqa in Syria, though some may have set up in desert hideouts along the border.

"They're not willing to share the risk that they demand of their fighters to fight to the death," U.S. Army Lt. Gen. Stephen Townsend, who heads the U.S.-led coalition against IS, said at a Pentagon press briefing.

Also escaping are many middle-rank IS fighters, the rough equivalent of a military's captains and majors crucial to keeping structure. A lieutenant-colonel with Iraqi intelligence estimated hundreds of IS fighters have fled west Mosul among the civilians. He spoke on condition of anonymity in line with regulations.

Those fighters could form the backbone of an IS rebirth as an insurgent force.

From the relative safety within the territory the Islamic State group still holds in Syria and the pockets remaining in Iraq, along with any sleeper cells the militant group has inside Iraqi government-controlled territory, IS can carry out insurgent operations in Iraq including suicide bombings, much as it did before 2014 in its incarnation as Al Qaeda in Iraq.

From Syria, the group can also plot attacks in the West.

"The Caliphate will not vanish," IS pledged in an internal publication found north of Mosul by Aymenn Jawad al-Tamimi, a researcher with the Middle East Forum who studies the group.

He said the document appeared to anticipate that urban areas like Mosul and Raqqa will be lost.

"It adopts the idea that the Caliphate does not end with loss of territory, and the West in particular should realize the next generation of the Caliphate's soldiers are being nurtured within their borders," al-Tamimi said.

WHAT'S NEXT IN THE FIGHT

The next likely target is Raqqa, in northern Syria. U.S.-backed Kurdish-led forces are trying to cut off supply lines into the city, and U.S. officials have said an assault could begin with weeks. There too, however, the militants are dug in for long and potentially grueling fight.

From there, the last stronghold of the militants could be the Deir el-Zour region in eastern Syria, near the border with Iraq.

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Iraqi forces will likely need weeks of rest and resupply after Mosul. But then the fight will also continue in Iraq to mop up the last pockets held by militants along the border. Also crucial to recapture are Iraqi border crossings still in the militants' hands, a step that would crimp — though not completely stop — IS's ability to move supplies and fighters into Iraq.

Senior military and intelligence officers warn that addressing Iraq's political divisions will be just as important.

In the past, the militants have been able to rebound from defeat by exploiting anger among Iraq's Sunni Arab minority, which feels marginalized by Shiites. And Iraq will have to deal with the monumental task of rebuilding cities destroyed in the fight against IS. Slowness in doing that could fuel Sunni resentment.

The U.S. ambassador to Iraq, Doug Silliman, said the next task will fall to the Iraqi government to return services to territory retaken from IS to deliver a lasting defeat to the group, at the Sulaimani Forum earlier this month.

"The military victories will not be enough," he said.