NATO's secretary general said there will be "severe consequences" if Russia fires nuclear weapons, as President Vladimir Putin has repeatedly threatened. The warning comes as U.S. forces and NATO allies conduct routine military drills in international waters, and as NATO's annual nuclear exercise is underway over northwestern Europe.

"It would be absolutely wrong if I went into the exact way we will react, but Russia knows that there will be severe consequences," Jens Stoltenberg told CBS News aboard the USS George H.W. Bush, a Navy aircraft carrier taking part in the defense drills. "We are vigilant and ready to react if something bad happens."

"Something bad" could be a "dirty bomb" — an explosive loaded with radioactive material. Russia claims Ukraine wants to use one on the battlefield, although there is no evidence to support the allegation. A "dirty bomb" is not a nuclear bomb, and would be less harmful, but by using conventional explosives to spread radioactive material, it is an effective weapon of terror.

Stoltenberg said the accusation could be a strategic tactic from Russia, foreshadowing what it aims to do instead.

"We have seen before that Russia has accused others for doing what they intend to do themselves," he said. "So therefore, we are sending a strong message to Moscow that they should not try to use a false pretext as an excuse for escalation of the war in Ukraine."

For now, NATO says its best weapon is overwhelming deterrence with a display of firepower from fighter jets, helicopters and surveillance. If called upon, the fighters are ready to strike, but only to defend the alliance and not to provoke Russia, the organization insists.

"We don't seek conflict with Russia," Stoltenberg said. "But of course, when we see more threats, when we see a more dangerous world, it's even more important that we send a clear message to any potential adversary that we are ready to defend all allies because that's the best way to prevent an attack and preserve peace."

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