The United States would almost certainly know in advance if Russia was preparing a nuclear strike on Ukraine, and Moscow may very well want the word out, nuclear weapons experts say.

Russian President Vladimir Putin has raised the possibility of using nuclear weapons if Russia’s “territorial integrity” or existence is threatened.

Moscow’s statement on Friday that it would annex four partially occupied regions of Ukraine potentially meant Russia could consider responding to attacks on the claimed territory with a nuclear strike.

Should such an escalation occur, experts say it would likely take the form of a smaller tactical nuclear weapon, likely launched on an Iskander short-range ballistic missile.

While military analysts are downplaying Moscow’s threats for now and US officials say they have seen no activity suggesting such plans, Western defense and intelligence agencies are watching closely for real nuclear threats emerging.

Preparations for an attack are obvious, Pavel Podvig, a senior researcher at the UN Institute for Disarmament Research in Geneva, told AFP.

A 2017 report by the institute lists 47 nuclear storage sites across Russia — 12 national-level facilities and 35 base facilities.

These are constantly monitored by US and foreign intelligence and military surveillance satellites.

They can even be observed closely by commercial satellites, as evidenced by the widely circulated, regularly updated images of activity at North Korea’s nuclear facilities.

Podvig says Russia has deployed its long-range strategic or nuclear warheads in the field, on missiles, bombers and submarines.

But its non-strategic or tactical nuclear weapons, of which there are up to 2,000, are stored and not installed in delivery vehicles like the Iskander, according to Podvig.

“There are no Iskanders walking around with nuclear warheads. Those guns are in storage,” he said.

“I am confident that the United States would see Russian preparations to use nuclear weapons,” said Mark Cancian, a former US Department of Defense and Energy official who worked on nuclear weapons issues.

“The weapons have to come out of storage, the units involved have to be alerted, and the Russians could also alert their strategic nuclear forces,” Cancian, now at the Center for Strategic and International Studies in Washington, told AFP.

He said evidence would also come from likely visible preparations for Russia’s ground forces, which would issue them protective gear and instructions on how to behave in a nuclear environment.

“All of this would be visible,” he remarked.

Podvig said that Moscow, like Washington, has for decades embraced the need for disciplined management of its nuclear warheads, and that system is quite strong and visible.

“We can be fairly certain that there are no hidden facilities,” he said.

“Nuclear weapons need a specific structure, trained people and maintenance. You can’t do that just anywhere.”

“Technically, you could probably smuggle a couple of bombs out of a camp smuggle undetected,” he said.

However, this carries risks, including provoking a pre-emptive attack from the West.

“The Russians will never be sure that it will go undetected. That would be a gamble,” said Podvig.

In addition, it is more likely that Russia would like the West to take its preparations as a warning.

“That would be a kind of escalation step (and) Russia would want that to become visible,” said Podvig.

For weeks prior to the February 24 invasion, the United States warned that Russia intended to attack Ukraine in a bid to prepare Kyiv and its allies — and potentially prevent Moscow from acting.

Would Washington openly warn the world if it discovered Russia was planning a nuclear attack? This could trigger an unprecedented panic, not only in Ukraine but also in other areas that could be hit by radioactive fallout.

Such an alarm could spread worldwide if people expected an escalation to a transcontinental nuclear war.

The United States would almost certainly warn allies and other powers, chief among them China and India, in hopes that it would pressure Moscow to withdraw or face international isolation.

According to Podvig, however, Washington would likely see issuing public warnings as useful in increasing pressure on Russia.

“The strategy must be based on isolation. It must be stressed that this is unacceptable, that it is criminal,” he said.

“This news could have chilling value.”