Pentagon claims significant victory against ISI S in Syria and Iraq
The Defense Department had previously estimated that IS fighters had lost control of about 40 percent of the territory they claimed in Syria and about 10 percent of the land they held in Syria.
"The number right now in Iraq is about 45 percent of the territory they once held has been recovered," Cook said.
"The number in Syria is anywhere between 16 to 20 percent."
IS jihadists stormed across large parts of Iraq and Syria in early 2014, meeting little resistance from Iraqi security forces and exploiting the chaos in civil-war-torn Syria.
Since August 2014, the United States has led an international coalition fighting back against the IS group, using a combination of air strikes and training and equipping local partners.
IS fighters have lost control of Ramadi and Heet in Iraq, but still control other important cities including Mosul and Fallujah.
In Syria, the group maintains control of Raqa, the capital of their so-called caliphate.