MINOT AIR FORCE BASE, N.D. — Defense Secretary Ash Carter said Monday the Pentagon is committed to correcting decades of short-changing its nuclear force, including forging ahead with building a new generation of weapons that will cost hundreds of billions of dollars in the coming decades.

In his first nuclear-focused speech since taking over the Pentagon in February 2015, Carter implicitly rejected arguments for eliminating any element of the nuclear force or scaling back a modernization plan that some consider too costly.

With the nose of a B-52 bomber at his back, Carter told airmen that the credibility of the American nuclear arsenal is crucial to ensuring its deterrence power. That credibility, he said, is built on personal performance.

“The confidence that you’re ready to respond is what stops potential adversaries from using nuclear weapons against the United States or our allies in the first place,” he said.

Earlier he flew by helicopter to a Minuteman 3 missile field and was taken 85 feet underground to a launch control center where two airmen are always present — 24 hours a day, seven days a week, 365 days a year — and ready to execute a presidential order to launch. The nuclear-tipped Minuteman 3 can reach a target on the other side of the globe in about 30 minutes.

It was Carter’s first visit to a nuclear weapons base as defense chief.

In his speech, Carter argued that even though the Cold War is long over, nuclear weapons are still needed to deter Russian and other potential aggressors from thinking they could get away with a nuclear attack.

“It is a sobering fact that the most likely use of nuclear weapons today is not the massive ‘nuclear exchange’ of the classic Cold War-type, but rather the unwise resort to smaller but still unprecedentedly terrible attacks, for example by Russia or North Korea,” he said.

He sketched an international security landscape dotted with nuclear dangers. He accused Russia of “nuclear sabre rattling” and North Korea of nuclear and missile provocations.

“A diverse and dynamic spectrum of nuclear threats still exists,” he said, adding that this makes it imperative that the U.S. ramp up its plan to modernize its nuclear weapons.

“We’re now beginning the process of correcting decades of underinvestment in nuclear deterrence — and I do mean decades, because it dates back to the end of the Cold War.”

He said the Pentagon plans to spend $108 billion over the next five years to sustain and improve its nuclear force.

“Even in 2016, deterrence still depends on perception — what potential adversaries see, and therefore believe, about our will and ability to act,” Carter said.

He did not mention directly the numerous morale, training, discipline and leadership problems that have beset the nuclear force in recent years, especially among those who operate, maintain and protect the Minuteman 3. One-third of the Minuteman force is based at Minot.

In what appeared to be an allusion to those issues, Carter stressed the importance of ensuring that every part of the nuclear “is working as smoothly as it should be.”

“So everyone playing their part is tremendously important,” he added. “It’s a mission that demands unparalleled excellence.”

After The Associated Press documented a range of problems in the Minuteman force starting in 2013, the Air Force began implementing what it calls a “force improvement plan” to boost morale, increase resources and attempt to eliminate the stigma that had become attached to the nuclear missile career field, which many saw as a dead end and much less rewarding than being a pilot.

Lt. Col. Jared Nelson, a Minuteman squadron commander at Minot, said in an interview Monday that he believes that perhaps 85 percent of the problems he saw at Minot a few years ago have been fixed. But the stigma, he said, is not gone.

“It’s not. Not by a long shot,” he said.