Biden Says Russian Use of a Nuclear Weapon Would Be a ‘Serious Mistake’

Just hours before Mr. Biden spoke at the White House, Ukraine responded, accusing the Russians of secretly building dirty bombs themselves at Europe’s largest nuclear power plant, which Russian troops now occupy.

American intelligence officials are divided about Russia’s intentions. Some believe that the repeated threats to use nuclear weapons are a bluff; others say they are part of a Russian military doctrine called “escalate to de-escalate,” in which a small nuclear device is set off to warn adversaries to stay away.

Over the past few weeks, the Biden administration has been conducting tabletop exercises, trying to game out how Russia might try to gain an advantage by threatening to use a nuclear weapon — and under what conditions it might actually detonate one. The purpose of the exercises, officials say, is to figure out how the United States and its allies might respond.

No one in the administration is arguing for a nuclear response. But among the options under debate are conventional military strikes on Russian forces inside Ukraine, likely executed by Ukrainian forces. The United States and its NATO allies would use the moment to further isolate Russia from the world — especially China and other nations, like India, that have been continuing to buy its oil. But it is unclear how Beijing in particular would respond to a Russian use of a small nuclear weapon, no matter how destructive.

On Tuesday, Mr. Biden was asked by a reporter whether Russia “is preparing to deploy a dirty bomb itself or a nuclear weapon.” The president focused on the second part of the question. While officials say a dirty bomb would be a tragic escalation, their clear concern is what it could portend. In American intelligence agencies and the Pentagon, there is fear Russia would stage a provocation to justify using a nuclear weapon in response.

American officials appeared to have some intelligence that backed up the fear, but they refused to discuss what it is, or how convincing it is. But in making public Russia’s claims to Mr. Austin and General Milley, they appeared to be following a pattern of making information public in an effort to box in Mr. Putin as he looks for options beyond relying on his failing military force.

“Russia is pushing transparently false allegations,” Karine Jean-Pierre, the White House press secretary, told reporters on Tuesday. “In the past, we’ve seen Russia use allegations as a pretext to escalate. And as the president said, we don’t know if that is the case here.”