"We have to defeat ISIL, we will defeat ISIL," Carter said using another acronym for ISIS in an interview with MSNBC's "Morning Joe." "It is something that must be defeated."
Carter stressed that the U.S. strategy must include defeating the group at its home base in Iraq and Syria "because that is the beating heart, that is the parent tumor of this thing called ISIL."
"At the same time we have to recognize that it has metastasized outside. That's where catching foreign fighters, identifying them taking them off the battlefield, dealing with finances, improving intelligence, getting more in the game, controlling borders," Carter said. "But the heart of it has to be defeating them in Syria and Iraq."
Carter also said he agreed with French President François Hollande that France and the U.S. are "at war" with the terror group, a fact that top officials in President Barack Obama's administration have acknowledged since the launch of the U.S.-led coalition bombing campaign more than a year ago.
Carter also sought to dispel criticism in the wake of the Paris terror attacks that the Obama administration is not doing enough to combat ISIS, pointing to the 3,500 U.S. troops on the ground in Iraq and the U.S.'s daily air sorties, which he called "very intense and very effective."
Carter said the U.S. is continually changing its tactics in the fight against ISIS and said he hopes European countries "do more than they have done so far" to help the coalition fight against ISIS.
But as France ramps up its involvement in the region, Carter said he is "glad the French are galvanized in joining the fight now."
Most Republicans have called for more aggressive U.S. military involvement in Iraq and Syria to defeat ISIS, including the deployment of ground combat troops, which Obama has strongly resisted.
Carter said that while Obama has "indicated he's prepared to do more, including on the ground," the U.S. needs to ensure it is building up and allowing local forces to take the lead in the ground fight against ISIS so that the group can "stay defeated" once it is.
"And that means that there have to be capable and motivated local forces that are prepared to sustain the defeat. We know from Afghanistan and we know from Iraq that that's the hard part," Carter said.