Iraqi civilians walk on the streets of a recently liberated neighborhood in western Mosu on Monday. Iraqi troops have surrounded Islamic state fighters who are bracing for a last stand in Mosul against the U.S.-led coalition.

MOSUL, Iraq — 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 its 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 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,” said Brett McGurk, the special presidential envoy for the U.S.-led coalition.

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 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.

Airwars, an independent group that tracks casualties, said several hundred civilians have been killed in March alone.