Working from this sun-scorched desert base, U.S. and allied commanders are beginning perhaps the most perilous phase of their fight against Islamic State: an attempt to recapture Mosul, Iraq's second-largest city, from the entrenched militant forces.

Military officers here say a barrage of airstrikes over the last two weeks helped sever two crucial routes that the extremist militants used to funnel fighters and supplies from the Syrian border to Mosul, their self-declared capital in Iraq and most significant battlefield prize.

U.S. commanders who help oversee the air war say the joint offensive with Iraqi Kurdish ground forces pushed back the Sunni Islamists' defensive line west of Mosul, recapturing territory and removing a key obstacle, at least for now, as military planners consider tactics for retaking the congested city as early as this summer.

American and allied advisors are training and equipping Iraqi security forces expected to lead any major ground assault. But options appear limited given the woeful state of Iraq's army, White House resistance to any plan likely to cause heavy civilian casualties, and at least some support in the Sunni-dominated city for the occupying force.

Although President Obama has repeatedly vowed not to reintroduce U.S. ground troops to Iraq, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told Congress in November that he would consider deploying a limited number of U.S. Special Forces to help direct airstrikes and assist the Iraqi army in any assault on Mosul.

Retaking the sprawling city, home to about 1.4 million people, almost certainly would require urban combat against a tenacious foe.

"Fighting inside a city like that will definitely not be easy, going street by street, house to house," said Ferhang Asandi, a Kurdish military officer. Islamic State militants are "trying to put all their effort and their fight in anticipation of the battle because they know that if Mosul is done it means the end" of a major source of prestige and recruitment.

The stakes are equally high for Iraqi Prime Minister Haider Abadi's fragile government in Baghdad, which has struggled to rebuild its army since entire divisions collapsed before the insurgent onslaught last year. Another military defeat at Mosul would undermine government authority and shift the momentum back to Islamic State.

"There is no way to create any form of unified or stable Iraq as long as Mosul is in hostile hands," said Anthony Cordesman, a senior military analyst at the nonpartisan Center for Strategic and International Studies in Washington. "Lesser victories are only a prelude to retaking Mosul."

Warplanes from the U.S.-led coalition have dropped more than 6,000 bombs on Islamic State positions in Iraq and Syria since August. Combined with attacks by Iraqi government forces, Kurdish fighters and Sunni Arab tribesmen who oppose the Islamist group, the campaign has stopped the militants from seizing much new territory and pushed them back in several areas.

In the last week, Kurdish fighters backed by hundreds of coalition airstrikes broke a four-month Islamist State siege on Kobani, a Syrian town on the border with Turkey. In Iraq, pro-government Shiite militias claimed they had taken "complete control" of Diyala province, northeast of Baghdad, after seven months of fighting the Sunni extremists.

But Islamic State has not been dislodged from any of the cities that it captured during its blitz last year.

Mosul fell quickly in June when convoys of heavily armed Islamic State fighters waving black flags stormed out of neighboring Syria and overran much of western and northern Iraq. Declaring an Islamic caliphate. the insurgents used the city to launch an offensive that pushed nearly 250 miles south to the edge of Baghdad before government forces and Shiite militias rallied to stop them.

Many residents of Mosul, who faced discrimination from the Shiite-dominated central government and military, initially welcomed the Sunni invaders. The militants seized vast arsenals of U.S.-supplied arms and munitions and hundreds of armored vehicles from fleeing government troops.

In the deadliest single battle after the 2003 U.S.-led invasion of Iraq, U.S. Marines fought for six weeks in late 2004 to oust entrenched Sunni insurgents from Fallouja, a city less than a third the size of Mosul. The battle saw the heaviest urban combat for the U.S. military since Vietnam. More than 90 Americans were killed and nearly 600 were wounded. Fallouja fell to Islamic State fighters early last year.

A battle in Mosul could be a tougher fight.

Kurdish officials say the militants already have reinforced their fighters, blocked roads and blown up a key bridge on the city's western edge to augment their defenses. They also have forged alliances with former Iraqi military officers and local Sunni officials who supported autocrat Saddam Hussein before he was toppled in the 2003 invasion.

"We've received information that they are creating fortifications, digging trenches around the city," said Jabar Yawar, spokesman for the Kurdish armed forces in Irbil, capital of the semi-autonomous Kurdish region in Iraq. "Also in every area they control, it is their custom to put bombs and booby traps."

An attacking force would need supporters or informants in the city for intelligence on the insurgents' positions and defenses. Although many Mosul residents complain of harsh edicts enforced by the extremists and shortages of basic commodities, the discontent does not appear close to a popular revolt.

For now, Iraq's security forces are concentrated on protecting Baghdad and surrounding areas. In December, heavy clashes were reported west of the capital in Ramadi, capital of Sunni-dominated Anbar province. Photos posted by the militants showed their fighters using captured armored personnel carriers and firing rocket-propelled grenades in what appeared to be street-to-street fighting.

Iraqi Kurdish fighters known as peshmerga are far closer to Mosul. Starting on Jan. 21, they launched an offensive on both sides of the Tigris River north of the city and seized a strategic junction at Kiske on an east-west highway that links Mosul to the Islamist State-held towns of Tall Afar, Sinjar and the Syrian border.

The Kurds worked with the U.S. Air Force and other military strategists here at Al Udeid, the command post for the air war. After consulting on an attack plan, the peshmerga hit an array of insurgent positions near the Mosul dam, about 30 miles northwest of the city, forcing many fighters into the open.

"It was literally like kicking an anthill," said Air Force Col. Lynn "Woody" Peitz, deputy commander of the air operations center at Al Udeid. "A lot of dynamic targets developed out of that."

Coalition warplanes dropped precision-guided bombs on militant positions, weapons depots and armored vehicles. The Kurdish fighters ultimately took back about 300 square miles of territory, officials said, and cut the supply routes.

Pentagon officials say they are encouraged by the gains but are under no illusions about the difficulties in trying to retake Mosul.

"It's difficult terrain. It's a big city. And they are entrenched there," Rear Adm. John Kirby, the Pentagon spokesman, told reporters Tuesday. "And oh, by the way, the enemy gets a vote" in what happens.

Special correspondent Nabih Bulos in Amman, Jordan, contributed to this report.

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