(NewsNation Now) — U.S. President Joe Biden says Americans shouldn’t be worried about nuclear war with Russia, but as the crisis in Ukraine deepens, some experts fear the regional fight could turn into a potential worldwide conflict. 

Russian President Vladimir Putin on Sunday told his top defense and military officials to put nuclear forces in a “special regime of combat duty,” claiming it was a response to “aggressive statements” from NATO, along with economic sanctions imposed by the United States and other Western nations in recent days for his invasion into Ukraine. 

Ukraine was once home to thousands of nuclear weapons. At one point, it was the third-largest nuclear power in the world. But when the Soviet Union collapsed in 1991, Ukraine had agreed to give up the weapons in exchange for a Russian and Western security guarantee, also known as the Budapest Memorandum.

The United States and Russia have the two largest nuclear arsenals in the world, by far. Russia has an estimated stockpile of about 6,257 nuclear warheads, compared to 5,600 in the United States, according to the Nuclear Information Project at the Federation of American Scientists. However, there is no evidence that either side has loaded any weapons onto any planes or submarines.

The turn of events is all the more remarkable for the fact that less than a year ago, Putin and Biden issued a statement at their Geneva summit that seemed more in keeping with the idea that the threat of nuclear war was a Cold War relic.

“Nuclear war cannot be won and must never be fought,” they agreed.

Pentagon press secretary John Kirby on Monday called Putin’s decision to put his strategic focus on high combat alert “unnecessary as it is escalatory,” but said the U.S. is confident in its ability to defend itself and allies against any threat.

“Secretary (Lloyd) Austin is comfortable with the strategic deterrence posture of the United States,” Kirby said, “and our ability to defend the homeland, our allies, and our partners.”

The Biden administration says they see no reason to change the current U.S. alert level, in spite of Putin’s order. In Russia, as in the United States, the president has sole authority to order a nuclear strike.

The Associated Press contributed to this report.