Jim Michaels

USA TODAY

Islamic State fighters surrounded during the key battle for Manbij, Syria, last week agreed to surrender their weapons to U.S.-backed Syrian forces in return for safe passage out of the embattled city, a senior defense official said Tuesday. It was the first such agreement with the terror group.

The exhausted and demoralized militants were using civilians as human shields, which is why the U.S.-backed fighters agreed to let them flee Manbij after three months of intense fighting and near constant aerial bombardment by the U.S.-led coalition.

The U.S. official, who was not authorized to discuss battle details, said the Islamic State fighters surrendered after being surrounded by the Syrian Democratic Forces, fighters recruited and trained by the U.S.-led coalition.

The official said the agreement to let the Islamic State militants escape likely saved hundreds of civilians held by the fanatical fighters, even though it allowed some Islamic State militants to escape.

The militants turned their weapons over to Syrian Democratic Forces before leaving Manbij, the official said.

The enemy convoy of 100 to 200 fighters left the city last Friday under the watch of coalition drones to ensure that the militants did not regroup and try to return to the city.

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The agreement was reached as the militants continue to lose territory in Syria and Iraq. Manbij was a central clearing house for foreign fighters coming to fight in Iraq and Syria, and the capture of the northern Syrian city is an important step toward an eventual assault on Raqqa, the Islamic State's de facto capital in Syria.

The militants' surrender could damage the Islamic State’s reputation as a fierce fighting force that refuses to quit even in the face of superior numbers and firepower.

Their retreat also may reflect a strategy to preserve the group's numbers and become a more traditional guerrilla force rather than an occupying army.

The Islamic State “has shown time and time again it is capable of making highly pragmatic decisions,” said Jennifer Cafarella, an analyst at the Institute for the Study of War, a think tank in Washington, D.C.

If sectarian tensions resurface in newly liberated cities it could help set the stage for an Islamic State return. The Sunni group has sought to exploit sectarian conflict between Shiites, who are a majority in Iraq, and Sunnis who are predominant in areas the Islamic State invaded. Many Sunni's are unhappy with Iraq's Shiite-dominated central government.

U.S. advisers who support the Syrian Democratic Forces knew of the deal and agreed not to target the convoy with airstrikes as it left the city, according to the defense official.

U.S. advisers urged their allies on the ground to consider other options, but the forces stuck to their plan to give the militants safe passage, the official said.

“The U.S. does not have direct control over these participants,” Cafarella said. Rather, the U.S. military is supporting local forces in the battle against the Islamic State so U.S. ground combat forces won't be needed.

Army Col. Chris Garver, a military spokesman in Baghdad, said at a briefing Tuesday that an Islamic State convoy left Manbij and was not targeted because it was carrying civilians among the militants. He said the decision not to bomb the convoy was made by the U.S.-backed forces in Manbij.

Manbij fell after three months of intensive fighting. In July, coalition warplanes dropped more than 1,000 munitions — including bombs, rockets and strafing runs — on militants in the area.

Roughly 2,000 Islamic State militants were killed by airstrikes alone in the three-month Manbij offensive, according to the U.S. official.

Ground forces engaged in deadly street fighting that was often measured by how many houses they secured on a given day.

Militants had laced the city with improvised explosives and rigged entire buildings to detonate as they withdrew into the center of the city, the Pentagon said.

The U.S.-backed ground forces continued to push into the city from all sides during fighting, which grew deadlier as the militants retreated to the city center for a last stand.

They had no escape route and their decision to surrender may have been influenced by the death of fellow fighters in Fallujah, Iraq, the defense official said.

In June, coalition airstrikes killed at least 348 Islamic State militants and destroyed more than 200 vehicles when several big convoys tried to escape advancing Iraqi forces in Fallujah, a predominately Sunni city west of Baghdad.

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In Manbij, coalition surveillance drones watched the militants leave the city via a damaged bridge that had one lane open.

During the battle, coalition pilots used precision airstrikes to disable several lanes of the bridge, but commanders had decided early in the offensive to leave one lane passable.

Still, the lane was so full of war debris the convoy was led out of the city by an earth mover that pushed twisted wreckage off the road so the vehicles could pass.