That means that unless Putin or his commanders want the world to know in advance, the US may never know when Russian forces have swapped out conventional munitions for atomic bombs.
It is an increasingly vexing problem as Russian forces struggle to regain the momentum in Ukraine and signs grow that Putin is increasingly unpopular at home, especially after he ordered a limited military draft last week.
“We’re watching it more closely,” said a US government official with access to intelligence on Moscow’s nuclear forces and strategy who, like others interviewed for this article, was not authorized to speak publicly.
Recent efforts include tasking additional US and allied intelligence assets — in the air, space and cyberspace — and relying more heavily on commercial Earth-imaging satellites to analyze Russian units in the field that might be in position to get the nuclear order, the official said .
Another focus outside Ukraine is the Russian enclave of Kaliningrad, sandwiched between Poland and Lithuania, where the Kremlin has installed dual-use weapon systems and hypersonic missiles.
Over the past week, flight-tracking radar websites have shown multiple US Air Force RC-135 Rivet Joint electronic surveillance planes circling the city, ostensibly collecting data. In the past several years, Russia has upgraded its missile storage sites in Kaliningrad, stoking fears of a potential nuclear buildup in the territory.
Putin has made veiled references since the Russian invasion of Ukraine in February that he might resort to using nuclear or chemical weapons to change the course of the battle or if Russia itself is threatened.
However, those threats grew bolder last week when he said he was prepared to “use all the means available to us,” including “various weapons of destruction.”
“I’m not bluffing,” he added.
In response, the United States warned of “catastrophic consequences,” but has purposefully left open what exactly that means.
“We have communicated to the Russians what the consequences would be, but we’ve been careful in how we talk about this publicly, because from our perspective we want to lay down the principle that there would be catastrophic consequences, but not engage in a game of rhetorical tit for tat,” national security adviser Jake Sullivan said on Sunday.
On Monday, the Kremlin said it had “sporadic” talks with the United States about nuclear issues, in what was seen as a potential effort to alleviate the tense situation. Russia’s deputy foreign minister also appeared to try to downplay Putin’s latest rhetoric, insisting that Russia had no plans to use nuclear weapons.
But on Tuesday, as Moscow prepared to annex some 15 percent of eastern Ukraine following referendums among the large Russian-speaking areas, one leader issued another, more explicit nuclear threat.
“Let’s imagine that Russia is forced to use the most fearful weapon against the Ukrainian regime which had committed a large-scale act of aggression that is dangerous for the very existence of our state,” said Dmitry Medvedev, deputy chair of Russia’s Security Council, in a post on Telegram, Reuters reported.
“I believe that NATO would not directly interfere in the conflict even in this scenario,” he added. “The demagogues across the ocean and in Europe are not going to die in a nuclear apocalypse.”
A spokesperson for US Strategic Command said the group is “always on watch and ready to respond if needed.”
“We haven’t seen any evidence at this time that Russia will use nuclear weapons,” said Lt. Cmdr. Joshua Kelsey. “We take these threats very seriously, but we have not seen any reason to adjust our own nuclear posture at this time.”
Yet gaining advanced knowledge of any looming Russian attack would by definition be a hard task. Some two dozen Russian weapon systems can deliver both conventional explosives and low-yield nuclear warheads, said the first US official.
And public estimates are that Russia has more than 1,900 tactical nuclear warheads, also referred to as non-strategic nuclear weapons.
“That’s everything from cruise missiles to nuclear torpedoes to gravity bombs to intermediate-range ballistic missiles,” the official said. “All sorts of stuff.”
Intelligence agencies, the official said, are confident that Russia would not risk an all-out nuclear war by launching a massive attack on Ukraine or NATO countries.
“They’ll never use a strategic nuclear weapon,” said the government official. “They’ll never launch an ICBM or put a [Tu-95] bomber loaded with megaton-class warheads. What they’ll do is use a short-range weapon. They have warheads that we call micro-nukes, with tens to hundreds of tons of explosive yield.”
By comparison, the explosive yields of the nuclear bombs that the United States dropped on the Japanese cities of Hiroshima and Nagasaki at the end of World War II were in the range of 15 to 20 kilotons, or 15,000 to 20,000 tons.
“That’s still a big bomb,” the official said, referring to micro-nukes, but stressed that “you can focus on really small tactical targets. … You don’t have lots of radiation.”
Russia is also known to have lower-yield atomic weapons for battlefield use that are much more powerful, including in the kiloton range that are on par with or surpass the bombs dropped on Japan.
Top intelligence officials have been stepping up their warnings in recent months about Russia’s growing reliance on tactical nuclear weapons in its military strategy.
Avril Haines, the director of national intelligence told Congress in February that Russia “is expanding and modernizing its large, diverse, and modern set of nonstrategic systems, which are capable of delivering nuclear or conventional warheads.”
“Moscow,” she added, “believes such systems offer options to deter adversaries, control the escalation of potential hostilities, and counter US and allied troops near its border.”
US military commanders and intelligence experts hope that the first indication that Russia has decided to go nuclear in Ukraine won’t be a mushroom cloud.
“The administration has spent an awful lot of time during this whole Ukraine war process talking about how much insight they had about what Russia was planning and then what Russia was going to do once the war started,” said a former senior National Security Council official who still advises US Strategic Command.
The person said the administration’s comments indicate a reliance on a mix of intelligence-collection tools, ranging from human spies to eavesdropping techniques to discern if such an order had been given or what particular Russian units believed to have nuclear weapons training might be carrying it out .
“That suggests certain access — HUMINT, SIGINT and imagery — that are pretty good in terms of penetrating the Russian system,” the person said, using the shorthand for human and signals intelligence.
But what makes it exceedingly difficult is that Russia has 23 different dual-use weapon systems, many of which it has been using in Ukraine.
“If the Russians have it in their arsenal as a conventional weapon, you can be pretty safe in assuming it has a nuclear warhead that goes with it,” the person said. “Almost every single weapon the Russians have is nuclear-capable. If it’s an artillery system, if it’s an air-defense system, if it’s a torpedo, if it’s a cruise missile, it could have a nuclear weapon with it.”
The first US official cited as an example the Iskander short-range ballistic missile system, which can fire both conventional and nuclear warheads.
Still, there could be subtle indications that the nuclear option has been put in motion, such as particular units with the means of delivering a small nuclear device behaving out of the ordinary, like pulling back some forces or equipment but not others.
“We might think, ‘huh that’s a little bit different from the way they normally operate. They are sending in this one unit, but pulling everybody else back. That’s really different. That’s strange,’” the government official outlined one possible scenario.
Others think that the Putin regime may want to telegraph his intentions in the hope of gaining diplomatic leverage.
“I think that the Russians, if they are getting ready to do that, would be trying to signal that,” said Franklin Miller, a former veteran Pentagon official and National Security Council nuclear policy official in the George W. Bush administration. “They would, in an ostentatious manner, arrange for the nuclear rounds to come out of the special storage sites. They’d give us a hint that they’re moving ammunition from central storage sites to firing units. And then give us more time to think about it and worry.”
Miller, who is now a consultant at The Scowcroft Group, added that “you could, in theory, see them loading a weapon on an airplane or special activity of some sort around a medium-range missile launcher.” However, he believes “that’s less likely.”
Another scenario could involve only “scarce minutes” of advance notice, Miller said. “There would be some communications traffic that tipped us that something was going to happen with a special round.”
The former senior NSC official also agreed that “they may want us to see some of these things.”
But the first US official is not counting on much notice. “For those smaller nuclear weapons, we’re probably not going to know.”
Paul McLeary contributed to this report.