ISIS Has Lost 25 Percent of Territory It Once Held in Iraq, US Says Despite assessment, long fight still expected against the militant group.

 -- Iraqi and Kurdish forces have reclaimed more than 25 percent of ISIS-held territory inside Iraq, according to a U.S. assessment that also determined that Kurdish fighters are responsible for the majority of the territory retaken from ISIS in northern Iraq.

“We assess ISIL’s front lines have been pushed back in northern and central Iraq,” Pentagon spokesman Col. Steve Warren said at a Pentagon briefing today, referring to the militant group also known as ISIS. “ISIL no longer has complete freedom of movement in roughly 25 percent of populated areas of Iraqi territory where they once operated freely.”

The recaptured areas represent an area between 4,100 and 5,200 square miles or 11,000 and 13,500 square kilometers, Warren said. At its peak, ISIS was in control of 55,000 square kilometers in northern and western Iraq, Pentagon officials said.

“ISIL lost large areas where it was once dominant in the governance of Babil, Diyala, Nineveh, Salahadin and Kirkuk,” Warren said.

Warren said the term "freedom of movement" equates to losing territory “if they don’t have freedom of movement they don’t control it.” I

The assessment that 25 percent has been recaptured is a significant increase from previous assessments released in January that Iraqi and Kurdish security forces had retaken 700 square kilometers.

“Their caliphate is in the process of shrinking,” Warren told reporters, noting that areas represented in the assessment were of populated areas that had been taken from ISIS control.

But he cautioned that the assessment was “not an exact science,” pointing to what he described as the “fluid battleground” inside Iraq.

Just last week, Iraq launched a major offensive to retake the northern city of Tikrit with a combined force 25,000 Iranian backed Shiite militia members and Iraqi security forces. The U.S has still not provided any airstrike support for the Iranian-directed operation, Warren said.

Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told Congress earlier this week that Iranian-backed Shiite militias made up 80 percent of the force pressing towards Tikrit. Warren said there have still been no U.S. airstrikes in support of the Iraqi offensive.

CIA Director John Brennan described the battle for Tikrit as “a good example” of the effort to dislodge ISIS from areas they controlled. He said he expects an “intense battle” in the city given that “Tikrit is an urban area.”

“They’re not invincible, they can be stopped and that’s been proven,” Brennan said during an appearance today at the Council on Foreign Relations in New York. But Brennan also urged patience in the fight against ISIS, noting “this is going to take time. It’s going to be a long tough fight."

The CIA director said his agency was not working with the Iranians inside Iraq, though both countries have an interest in defeating ISIS.

Brennan said he thought there was “an alignment of some interest between ourselves and Iran, clearly in terms of what ISIL has done there.”

“So we work closely with the Iraqi government, the Iranians work closely with the Iraqi government as well,” he added. "So some of these efforts I think through the Iraqi interlocutors are ones that again are trying to advance our common objectives against ISIL.”