"In Iraq, it's about 40 percent," said Colonel Steve Warren, spokesman for the international coalition which carries out daily air raids against IS and also provides training and weapons to local forces fighting the group.
"In Syria... we think it's around 20" percent, he said.
When the size of the so-called caliphate IS proclaimed 18 months ago was at its largest, Iraq accounted for a slightly bigger part of it than Syria.
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"Taking together Iraq and Syria... they lost 30 percent of the territory they once held," Warren told reporters in Baghdad.
Since taking control of Ramadi in Iraq and Palmyra in Syria in May 2015, IS has been on the back foot most of the time.
A variety of Iraqi forces -- including the Kurdish peshmerga, the Shiite-dominated Popular Mobilisation paramilitary force and the federal forces -- have reclaimed major urban centres, including Ramadi last week.
Warren's estimate appear to differ significantly from a figure of 14 percent provided by the IHS Jane's think-tank, albeit predating the retaking of Ramadi.