Manbij Liberated After ISIS Fighters Flee City

— -- Defense Secretary Ash Carter has congratulated the anti-ISIS rebel fighters for liberating the Syrian city of Manbij this weekend, a victory accelerated by the departure of ISIS fighters who used civilians as human shields in a convoy of 500 vehicles.

"I want to congratulate the people of Manbij, the Manbij Military Council, the Syrian Arab Coalition and the Syrian Democratic Forces for their success in liberating Manbij city from ISIL's hateful rule," said Carter in a statement issued Monday.

"Their success, enabled by critical Coalition support, is another significant milestone in the campaign to deal ISIL a lasting defeat," said Carter. Located in northern Syria close to Turkey, the city was a main transit point for ISIS foreign fighters coming into and leaving Syria.

"The success in Manbij city will also help reinforce the growing isolation of Raqqa and enable us to achieve the next objective of our campaign in Syria -- collapsing ISIL's control over that city," said Carter.

For more than six weeks, the city had been surrounded by a mostly Syrian Arab force of 3,000 fighters that pushed into the city with the support of dozens of American airstrikes.

A U.S. official told ABC News that Manbij was liberated over the weekend after the remaining ISIS fighters fled the city using human shields to cover a convoy of about 500 vehicles.

According to the official, the vehicles were spotted gathering at a rally point in the city; the convoy then headed north through an ISIS-established humanitarian corridor. The ISIS fighters were interspersed in vehicles among the civilian vehicles, making it difficult to track which ones were carrying only ISIS fighters, civilians or a mix of civilians and fighters. ISIS had also warned the Syrian Arab Coalition and the Syrian Democratic Forces in advance that they would kill civilians traveling with them if they came under attack.

In late June, two large convoys of ISIS fighters fleeing Fallujah were destroyed by a large number of airstrikes. But similar airstrikes were not feasible outside Manbij because it was difficult to identify ISIS fighters inside the vehicles.

The official said that after the convoy headed north, ISIS fighters broke off from the civilians and dispersed. The majority of the civilian vehicles then returned to Manbij.