President Biden renewed his warning to President Vladimir V. Putin that it would be an “incredibly serious mistake” to use a tactical nuclear weapon in Ukraine, reflecting the increasingly urgent concern in Washington and among Western allies that Russia may be searching for a pretext to unleash such a weapon.
Mr. Biden said on Tuesday that he was still uncertain if Russia was trying to put together a “false flag operation” in which it would detonate a dirty bomb and blame the Ukrainians. A dirty bomb is not a nuclear weapon, but an improvised device that uses conventional explosives to spread radioactive material.
But it was clear from Mr. Biden’s comments that he is far less concerned about a dirty bomb than about the possibility that a set of incidents could result in Russia detonating a battlefield nuclear weapon, the first to be used in a conflict since the United States dropped atomic weapons on Hiroshima and Nagasaki.
“Let me just say: Russia would be making an incredibly serious mistake for it to use a tactical nuclear weapon,” Mr. Biden said in response to a reporter’s question about whether Russia was preparing a dirty bomb. “I’m not guaranteeing you that it’s a false flag operation yet, don’t know, but it would be a serious, serious mistake.”
Mr. Biden’s comments, and a complex exchange of accusations between Russia and Ukraine, were the latest demonstration of how high tensions are running at a moment when Russia is losing ground and sees no pathway to controlling vast areas of the four provinces it recently annexed.
It is part of what American officials have called an inescapable paradox of the conflict: While the United States and its NATO allies are committed to helping Ukraine expel Russia from its territory, the more successful the Ukrainians are, the greater the risk that Russia will break the unwritten taboo against employing nuclear weapons.
Russia’s top military officials began the series of accusations about dirty bombs over the weekend, warning their American counterparts, Defense Secretary Lloyd J. Austin III and Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, that Ukraine was planning to detonate a radiological attack on Ukrainian soil. They offered no evidence.
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.
John Ismay, Jim Tankersley and Noah Weiland contributed reporting.