Russian President Vladimir Putin chairs a Security Council meeting via a video link in Moscow on September 23, 2022.
Washington CNN  — 

US officials believe that the likelihood Russian President Vladimir Putin will use a tactical nuclear weapon in his struggling war in Ukraine is perhaps the highest it has been since Russia invaded in February — but is still not probable, multiple officials familiar with the latest intelligence tell CNN.

The intelligence community is closely watching for any signs that Putin’s calculus has changed after the Russian president was widely perceived last week to be escalating his past threats to use nuclear weapons.

The threat is certainly “elevated” compared to earlier in the year, according to multiple sources. The US in recent months has been privately warning Russia not to take such a catastrophic step.

But so far, there are no signs that Russia is imminently planning their use and the “general assessment hasn’t changed,” one source familiar with the intelligence said.

Several US defense officials, who also said they see no indication at this time of Russia moving nuclear weapons around, said they believe it’s likely the US could detect movement even of smaller tactical warheads.

Officials have long believed that Putin would only turn to a nuclear weapon if there was a threat to his own position, or if he perceived an existential threat to Russia itself – which he may consider a loss in Ukraine to be.

Some Russian military analysts believe that Putin’s mobilization order may in fact decrease the short-term risk he will turn to a battlefield nuke, because it will prolong his ability to sustain the conventional war.

The general sense inside the US government that the threat is higher than before is based primarily on Putin’s rhetoric and analysis of his mindset amid Russian losses in Ukraine, rather than any hard intelligence that Russia is more seriously weighing the nuclear option, according to two sources familiar with the intelligence.

For example, some officials are concerned that Putin could take extreme steps to protect Russian-occupied territory in eastern Ukraine, amid sham referendums held there that are expected to result in Russia forcibly annexing the territory. Putin also said last week that Russia’s threats to deploy a nuclear weapon are “not a bluff.”

Still, the intelligence community’s view into Putin’s decision-making calculus is imperfect and multiple sources acknowledged that even a marginally higher probability of the use of such weapons is concerning. Russia’s dismal performance and Ukraine’s relative success in its recent offensive push in the northeast have left Moscow with a vanishing number of choices on the battlefield.

“It’s hard to track definitively if/when he would give such an order,” one of the sources said. “Or how his own mental calculus is playing out.”

Little concern Russia would use a ‘strategic’ nuclear weapon

There is little concern that Russia will use what is known as a “strategic” nuclear weapon — warheads that have explosive yields of 500 to 800 kilotons and are designed to destroy entire cities.

“Tactical,” or “battlefield” warheads — also known as “low yield” warheads — are designed for use in a battlefield setting. They have explosive yields of 10 to 100 kilotons of dynamite. But they can still be unimaginably deadly: the bombs dropped on Hiroshima and Nagasaki in Japan in 1945 were the equivalent of about 15 to 21 kilotons, respectively.

Perhaps the more important distinction, some analysts say, is how a warhead is used rather than its yield. Some officials – like former US Secretary of Defense Jim Mattis – believe the “tactical” v. “strategic” distinction is meaningless. Using either would cross a rubicon not traversed since 1945.

Officials familiar with the intelligence say Putin has a number of options in how he could choose to deploy such a weapon. He could test one at sea as a show of force and an effort to force concessions from either the west or Ukraine. Or he could move to use one inside Ukraine itself, either on the battlefield or in a population center — an option that several US officials said would instantly make him a pariah on the global stage. Some officials believe that even countries like China and India would move to isolate Russia if Putin took such a drastic step.

How the US responds would depend on which of those options Putin chose, President Joe Biden said in an interview with CBS’s “60 Minutes” earlier this month.

The US reaction would be “consequential,” but would depend “on the extent of what they do,” Biden said, without providing further details.

In a speech last week, Putin warned that, “In the event of a threat to the territorial integrity of our country and to defend Russia and our people, we will certainly make use of all weapon systems available to us. This is not a bluff.”

US officials have emphasized that this is not the first time Putin has threatened to turn to nuclear weapons since the start of his re-invasion of Ukraine in February, although some analysts have seen this threat as more specific and escalatory than the Russian president’s past rhetoric.

The US has also sought to deter Russia from using a nuclear weapon in public warnings in the past and made the issue a theme of remarks at the UN General Assembly this week in New York. Secretary of State Antony Blinken last week said Russia’s “reckless nuclear threats must stop immediately.”