According to the US Defence Department, it had previously estimated that IS fighters had lost control of about 40 percent of the territory it formerly claimed in Iraq, and about 10 percent of the land it held in Syria.
Pentagon spokesman Peter Cook said that the tallies had gone up in recent weeks.
“The number right now in Iraq is about 45 percent of the territory they once held has been recovered. The number in Syria is anywhere between 16 to 20 percent,” abc.net.au quoted him as saying.
The gains in Syria come as Turkey and the US-led coalition strike in IS targets are reported to be continuing successfully.
The United States has since 2014 led an international coalition fighting back against the IS group, using a combination of air strikes and training and equipping local partners.
Turkish State media has said that more than 20 IS militants were killed on Monday.
In early 2014, the IS stormed across large parts of Syria and Iraq exploiting the chaos situation in civil war-torn Syria and meeting little resistance from the Iraqi security forces.
The group has since lost control of Ramadi and Heet in Iraq, but still controls other important cities including Mosul and Fallujah.
In Syria, it maintains control of Raqa, the capital of their so-called caliphate.