Coalition forces have been hitting IS targets since August 2014 after the extremists seized huge parts of Iraq and Syria, and critics have accused the campaign of moving too slowly.
"Right now, without question, the momentum against ISIS is more than at any other time in our campaign," Deputy Defense Secretary Robert Work said, using an acronym for the IS group.
"We are more confident than ever that militarily, ISIS is on its way to a defeat on the battlefield."
But he warned that a lasting defeat required more than only military action.
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"There is going to have to be political accommodations which still have yet to be revealed," Work told an audience during a discussion hosted by The Washington Post.
At a separate event, Defense Secretary Ash Carter later pointed to recent successes against the IS group, including the targeting of high-up members and severing transit lines between the IS group's main power center in the Syrian city of Raqa and areas in northern Iraq.
"ISIL is now under pressure from all sides," Carter said.
"We will continue to accelerate this campaign and we will be doing more in accordance with our strategic approach and leveraging our unique capabilities."
IS-linked extremists have been blamed for both the November Paris attacks and last week's suicide bombings on Brussels airport and metro that killed 32 people.