"Our strategy right now is to accelerate the campaign against [the Islamic State]. It is a threat to all civilized nations," Mattis said in an interview that aired Sunday on CBS News' Face the Nation.
Last week, U.S. President Donald Trump told leaders during his first foreign trip since taking office that defeating terrorism is a priority. In Saudi Arabia on May 21, he called on Muslim nations to join the "battle between barbaric criminals who seek to obliterate human life, and decent people of all religions who seek to protect it."
Mattis said the "bottom line is we are going to move in an accelerated and reinforced manner, throw them on their back foot."
The defense secretary said strategy has changed.
"We have already shifted from attrition tactics where we shove them from one position to another in Iraq and Syria, to annihilation tactics where we surround them," Mattis said. "Our intention is that the foreign fighters do not survive the fight to return home to North Africa, to Europe, to America, to Asia, to Africa. We're not going to allow them to do so. We're going to stop them there and take apart the caliphate."
Once the Islamic State is annihilated, the allies must make sure another group doesn't take its place.
"Once ISIS is defeated," Mattis said, using an alternate name for the terror group, "there's a larger effort underway to make certain that we don't just sprout a new enemy," Mattis said. "We know ISIS is going to go down. We've had success on the battlefield. We've freed millions of people from being under their control, and not one inch of that ground that ISIS has lost has ISIS regained."
Mattis didn't give a timeline.
"This is going to be a long fight," Mattis said. "The problems that we confront are going to lead to an era of frequent skirmishing. We will do it by, with, and through other nations. We will do it through developing their capabilities, to do a lot of the fighting, we'll help them with intelligence. Certainly, we can help train them for what they face. And you see our forces engaged in that from Africa to Asia. But at the same time, this is going to be a long fight and I don't put timelines on fights."
Pictures from the scene were published in The New York Times and the name of the suspected attacker, Salman Abedi, was also released to the media. The New York Times did not reveal how it obtained the photos.
"I believe when you leak the kind of information that seems to be routinely leaked -- high, high level of classification... I think it's darn close to treason," U.S. Homeland Security Secretary John Kelly told NBC's Meet The Press.
"I don't know where the leak came from," Kelly said. "But I will tell you this, as I always do in cases like this, I immediately called my counterpart in the U.K. And after offering my condolences about the attack ... she immediately brought this topic up. And, if it came from the United States, it's totally unacceptable. And I don't know why people do these kind of things, but it's borderline, if not over the line of, treason."
The sharing of intelligence related to the case between British counter-terrorism police and U.S. officials was briefly paused due to the leak.