Defense Secretary Jim Mattis listens on Capitol Hill in June while testifying before the Senate Armed Services Committee about the Pentagon’s budget. (Jacquelyn Martin/Associated Press)

MINOT AIR FORCE BASE, N.D. — Defense Secretary Jim Mattis said Wednesday that he is now “convinced” that the Pentagon must maintain three ways to launch a nuclear attack, but that he could make cuts to specific kinds of weapons in the future.

The comments came amid an ongoing review of the U.S. nuclear weapons program and as tensions with North Korea remain high as it works toward developing a nuclear missile of its own. For decades, the U.S. military has been able to deliver nuclear weapons by Navy submarine, Air Force bombers and Air Force intercontinental ballistic missiles (ICBM), a system known as the triad and due for hundreds of billions of dollars in upgrades in coming years.

“If I want to send the most compelling message, I have been persuaded that is the triad, and its framework is the right way to go,” Mattis said, speaking on a military aircraft traveling from Washington.

Before becoming Trump’s defense secretary, Mattis questioned the need to keep all three “legs” of the triad. As a retired general, he testified as a national security expert before the Senate Armed Services Committee in January 2015 that the United States must clearly establish the role of nuclear weapons in U.S. policy, and that it was time to look at whether “to reduce the triad to a dyad.” Doing so, he said then, could reduce “false alarm danger” in which a nuclear war is set off by mistake.

But Mattis said he has concluded that the country must keep all three methods for delivering nuclear weapons, saying it poses the best chance of stopping an enemy from destroying the U.S. nuclear arsenal without facing a devastating counterattack. That framework, known as strategic deterrence, has existed for years as a cornerstone of U.S. national security policy.

That does not mean that all existing weapons programs are safe. Mattis cited as one example a proposed new cruise missile that would be delivered by air. The Air Force last month signed a contract worth up to $900 million with Lockheed Martin to begin developing the weapon, but Mattis said that was just to keep it for now as an option.

“It is not a decision yet,” Mattis said. “That will come out of the nuclear posture review.”

Mattis said that within each leg of the nuclear triad, there are types of weapons that he is reviewing.

“Does each need to be there? Are they stabilizing weapons? Are they necessary?” he asked, explaining his methodology. “You know, just because we had them 30 years ago, do we still need them?”

Mattis arrived on this remote Midwestern base as part of a three-day tour that also includes stops at the headquarters of U.S. Strategic Command in Omaha and Mexico City. The visit to Minot and a similar stop at Naval Base Kitsap-Bangor in Washington last month will help Mattis as he gathers information and makes judgment calls about what is needed in the future, he said.

After arriving, Mattis and his staff quickly embarked in Vietnam-era Huey helicopters that the Air Force uses to travel from Minot Air Force Base to surrounding missile sites. The secretary spoke privately with missile launch officers, visiting some of them in an underground capsule from which nuclear weapons can be launched, before meeting separately with airmen who perform maintenance on B-52 bombers and ICBMs.

On Wednesday evening, Mattis arrived at Offutt Air Force Base in Omaha, where U.S. Strategic Command oversees U.S. nuclear weapons and monitors potential nuclear threats. He is expected to receive several classified briefings just a few days after North Korea’s most recent nuclear test, in which Washington assessed that the North Koreans detonated a 100-kiloton nuclear bomb in an underground facility.

Mattis declined to say whether the North Korean device used was a hydrogen bomb, or whether the Pentagon is open to responding by putting its nuclear weapons in South Korea, which officials there have discussed. But Mattis added that the United States “has a pretty good idea” of what happened in the reclusive regime’s most recent nuclear test.