Defense Secretary Jim Mattis said Friday that the U.S. and South Korea are fully prepared to respond to an attack from North Korea while playing down the possibility of a first strike -- conventional or nuclear.

"We could fight tonight, shoulder-to-shoulder" with the South Koreans, Mattis said, but only "if they're attacked."

On Wednesday, a report in the Business Insider said that the recent deployment of B-2 Spirit Stealth bombers to Guam and upgrades to the weapons stockpile "suggest preparation for a potential tactical nuclear strike" against North Korea to destroy its nuclear and intercontinental ballistic missile sites.

"That's completely untrue," said Hans Kristensen, director of the Nuclear Information Project for the Federation of American Scientists. "There's no indication that's happening."

The published report cited improvements to lower the yield of the B61 nuclear gravity bomb, but the weapon has yet to be deployed.

The U.S. has a range of military options from the conventional to the nuclear, Mattis said, but "they exist so that the diplomats speak from a position of authority, that they have to be listened to, because an attack on the Republic of Korea will be severely rebuffed if it's attempted."

The bottom line is, "This is still in the diplomats' hands, as you know," Mattis told reporters on his plane to Hawaii, where he was meeting with South Korean Minister of Defense Song Young-moo.

The U.S. and other nuclear powers constantly review their strategies, and the U.S has never ruled out a first strike.

However, the suggestion that the U.S. is actively readying a tactical nuclear strike against North Korea was irresponsible, Kristensen said.

"There are always strike plans, contingency plans," he said.

Clarifying Mission

Officials at Pacific Command also clarified the B-2's role in its recent deployment.

"The B-2 Spirits are deployed in support of U.S. Strategic Command's bomber assurance and deterrence mission -- a regular rotation to support [Pacific Command]-led air operations, ensuring bomber crews maintain a high state of readiness and crew proficiency," Lt. Col. Lori Hodge of Pacific Air Forces told Military.com in a statement on Friday.

Hodge said that while the command could not comment on operations for operational security reasons, "it is fair to characterize bomber assurance and deterrence deployments as relatively short in duration -- weeks."

"The 36th Wing's ability to host and employ all three of the U.S. Air Force's bomber aircraft demonstrates Andersen Air Force Base, Guam's role as one of our nation's premier power projection platforms in the Pacific theater, last hosting all three bombers in August 2016," she said, referring to the latest, simultaneous B-52H Stratofortress and B-1B Lancer deployment in the region.

The long-range B-52 bombers replaced the B-1Bs in an 18-month-long mission in the Pacific earlier this month. Hodge said that changeover is now formally complete.

'A Nuclear Weapon Is a Nuclear Weapon'

As the Business Insider report stirred controversy, George Shultz, the former secretary of state under President Ronald Reagan, warned against the development of low-yield nuclear weapons in testimony Thursday to the Senate Armed Services Committee.

"A nuclear weapon is a nuclear weapon," he said. "You use a small one, then you go to a bigger one. I think nuclear weapons are nuclear weapons, and we need to draw the line there."

Shultz said he feared that the standoff with North Korea, and concerns about Russia's modernization of its arsenal, would make the first-strike option seem more feasible to the Trump administration.

"These weapons are immoral, as President Reagan said many times," he said in stressing that low-yield nuclear weapons will not make the U.S. any safer.

"Your mind goes to the idea that, yes, nuclear weapons become usable. And then we're really in trouble, because a big nuclear exchange can wipe out the world," Shultz said.

On Tuesday at the American Enterprise Institute, CIA Director Mike Pompeo said on the subject of North Korea, "The president is intent on delivering a solution through diplomatic means."

He added, "We are equally, at the same time, ensuring that if we conclude that is not possible, that we present the president with a range of options that can achieve his stated intention" of realizing the long-term U.S. goal of denuclearizing the Korean peninsula.

-- Richard Sisk can be reached at Richard.Sisk@Military.com.

-- Oriana Pawlyk can be reached at oriana.pawlyk@military.com. Follow her on Twitter at @Oriana0214.