WASHINGTON — Senior Russian military leaders recently had conversations to discuss when and how Moscow might use a tactical nuclear weapon in Ukraine, contributing to heightened concern in Washington and allied capitals, according to multiple senior American officials.
President Vladimir V. Putin was not a part of the conversations, which were held against the backdrop of Russia’s intensifying nuclear rhetoric and battlefield setbacks.
But the fact that senior Russian military leaders were even having the discussions alarmed the Biden administration because it showed how frustrated Russian generals were about their failures on the ground, and suggests that Mr. Putin’s veiled threats to use nuclear weapons might not just be words.
Still, American officials said they had seen no evidence that the Russians were moving nuclear weapons into place or taking other tactical measures to prepare for a strike.
The intelligence about the conversations was circulated inside the U.S. government in mid-October.
U.S. officials would not describe the scenarios the military leaders considered for use of a nuclear weapon. However, William J. Burns, the C.I.A. director, has previously said that Mr. Putin’s “potential desperation” to extract a victory in Ukraine and setbacks in the war could lead Russia to use one.
John F. Kirby, a National Security Council official, declined to comment on “the particulars of this reporting.”
“We’ve been clear from the outset that Russia’s comments about the potential use of nuclear weapons are deeply concerning, and we take them seriously,” Mr. Kirby said. “We continue to monitor this as best we can, and we see no indications that Russia is making preparations for such use.”
The Pentagon estimates that Russia has a stockpile of as many as 2,000 tactical nuclear weapons, which are designed to be used on battlefields to overwhelm conventional forces. No tactical nuclear weapon has ever been used in combat, but one could be deployed any number of ways, including by missile or artillery shell.
Tactical nuclear weapons carry lower yields and are meant to be used at shorter ranges than the warheads carried on intercontinental ballistic missiles.
Military experts say the use of a nuclear weapon — for the first time in more than 75 years — would fundamentally change the shape of war. Although the resulting destruction would depend on many factors, including the weapon’s size and the winds, even a small nuclear explosion could cause thousands of deaths and render parts of Ukraine uninhabitable.
Mr. Putin has sole authority over whether to use a tactical device and would make the decision to deploy one regardless of the views of his generals.
The new intelligence surfaced when Moscow was also promoting the baseless notion that Ukraine was planning to use a so-called dirty bomb — a conventional explosive laced with radioactive material. And it came amid a flurry of contacts between U.S. and other Western officials and their Russian counterparts, including two calls between Defense Secretary Lloyd J. Austin III and the Russian defense minister, Sergei K. Shoigu.
While the risk of further escalation remains troublingly high, Biden administration officials and U.S. allies also say that the phone calls between Western and Russian counterparts late last month helped ease some of the nuclear tensions. A speech by Mr. Putin last Thursday in which he denied that Moscow was preparing to use a nuclear weapon in Ukraine further lowered the temperature, according to some of the officials.
“We see no need for that,” Mr. Putin said in his speech. “There is no point in that, neither political, nor military.”
Since Russia invaded Ukraine in February and suffered significant casualties, Mr. Putin himself has fanned fears that he might resort to a nuclear weapon.
A European official said the speech was seen among allies as part of Mr. Putin’s regular pattern of inflaming tensions, watching the reaction of the West and his own public, then taking steps to calm the situation.
Russia conducted an annual military exercise last week testing nuclear-capable missiles. Mr. Austin said U.S. officials did not believe the maneuvers were “some kind of cover activity” to use a tactical nuclear weapon against Ukraine.
In Washington, administration officials say they still do not think Mr. Putin has made plans to use a tactical nuclear weapon or even a dirty bomb.
“We have not seen anything to indicate that Putin has made a decision to use a dirty bomb,” Mr. Austin told reporters at the Pentagon last Thursday. He said that even talk of the use of nuclear weapons was “dangerous.”
But he said the administration was “certainly concerned about escalation,” as it has been since the war started.
“It would be the first time a nuclear weapon has been used in over 70 years,” he said. “If this happened, we have been clear from the very beginning that you would see a very significant response from the international community.”
Biden administration officials have steadfastly refused to publicly describe what that response would be, but the president has indicated he has no plans to retaliate with an American nuclear device.
“I’m the guy that makes the recommendation to my boss on what we should do and how we should do it, and so I’ll make sure that he has credible responses that are actually effective in terms of what we want to do,” Mr. Austin said, without elaborating.
For Mr. Putin, using a smaller-yield, tactical nuclear weapon in Ukraine presents more complications than ordering the firing of a strategic weapon, like an ICBM. Moving a tactical nuclear weapon is not simply a matter of giving an order and having two people turn keys.
There would be practical steps that Russian commanders would be alerted to be ready to carry out, including how to mitigate any risk to Russian military personnel in the blast area.
“If there’s an order from the Kremlin, it can’t just magically happen,” Peter B. Zwack, a retired one-star Army general who was the American defense attaché in Moscow from 2012 to 2014, said in an interview last week.
From the beginning of the war, American intelligence agencies have been looking for signs that Mr. Putin is taking preparatory steps to use a nuclear weapon, such as undeclared nuclear exercises or strategic forces being put on alert.
But American officials said that the warning systems to detect such steps were imperfect, and that there was no guarantee that military or intelligence officials would be able to give the White House much advance warning. That is one reason the intelligence about the Russian discussions was viewed with such interest.
With the high-level nuclear discussions, the Russian military is weighing what might prompt Moscow’s use of a tactical weapon. And any additional intense discussion among senior leaders about the use of a nuclear weapon is something American officials have been profoundly concerned about — especially if Russia’s army in the south of Ukraine were to collapse.
Tensions over nuclear weapon use have been rising steadily since Ukraine’s successful counteroffensive in early September.
Since then, Mr. Putin has taken steps to escalate the conflict — calling for a mobilization, annexing territory in Ukraine, getting more directly involved in war planning, and approving a campaign to attack and degrade the electrical power grid of Ukraine with cruise missiles and Iranian drones.
Those steps have not changed Russia’s fortunes, and Ukrainian forces continue to move forward on fronts in the northeast and the south.
Still, some of those escalatory moves, like bringing in more troops, could have more impact on the battlefield by early next year.
Officials expect intense fighting to continue into next month, but muddier conditions and colder weather in the next few weeks could force a pause into earlier next year.