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ISIS has lost nearly a quarter of its territory over the past 15 months, according to a new data from a leading research company — the latest sign that the militants' once rapid advance has slowed.

Data published by IHS on Wednesday showed that since January 2015 the militant group has lost 22 percent of its territory in Iraq and Syria — and 8 percent of the losses were in the past three months.

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"The tide of the war is turning against" ISIS, said Columb Strack, senior analyst at IHS' Conflict Monitor. He added in a statement that the extremist group was "increasingly isolated, and being perceived as in decline."

The "major losses" for ISIS in Syria come as the group's domestic rivals have made advances under the cover of U.S. and Russian airstrikes, according to Strack.

Related: Name, Age, Fighting Skills: ISIS Personnel Files Leaked

The IHS data was published the day after State Department spokesman John Kirby said there had been "more and more defections" from ISIS in recent weeks.

"Fighters are becoming disenfranchised... and are in increasing numbers choosing to leave the group," Kirby told reporters, adding that the jihadis were increasingly reliant on child soldiers to battle "side-by-side with adult fighters."

ISIS overran huge areas of Iraq and Syria in summer 2014. In January this year, coalition spokesman U.S. Army Col. Steve Warren said ISIS had lost about 40 percent of the territory it had once held in Iraq and 20 percent in Syria.