The US intelligence agencies are yet to see any "practical evidence" that Russian President Vladimir Putin will resort to nuclear weapons in Ukraine, CIA Director William Burns said Saturday after Ukrainian President Volodymyr Zelensky claimed that his counterpart in Moscow could use such weapons.

"We don't see, as an intelligence community, practical evidence at this point of Russian planning for the deployment or even potential use of tactical nuclear weapons," Burns told a Financial Times conference in Washington, reiterating a previous assessment of his made in early April.

Burns, however, still made an assertion that Putin "doesn't believe he can afford to lose," stressing that the US should up its caution regarding the "nuclear threat" regardless.

Pentagon spokesperson John Kirby had also made the same statements a while back, calling Moscow's rhetoric on the potential use of nuclear weapons dangerous, also revealing that officials in the Department of Defense "haven't seen anything that would lead us to conclude that we need to change our strategic deterrent posture."

Moscow has repeatedly said Russia would not use nuclear weapons against its neighbor. "Russia firmly abides by the principle that there can be no victors in a nuclear war, and it must not be unleashed," Foreign Ministry spokesperson Alexei Zaitsev said Friday.

The Western claims come in light of Russia placing its nuclear deterrence forces on high alert at the onset of the Ukraine war and President Putin warning that foreign powers interfering with Russia's goals would face consequences "never seen in [their] entire history."

Those two factors came together to "convince" Zelensky that Russia could use chemical or nuclear weapons to pull off a victory in Ukraine, nudging the world to "be ready" for the possibility.

Ahead of Friday's reiteration that Moscow would never resort to deploying nuclear weapons, the Russian ambassador to the United States, Anatoly Antonov, stressed Thursday that it was his country "that in recent years has persistently proposed to American colleagues to affirm that there can be no winners in a nuclear war, thus it should never happen."

In late March, just over a month after the war broke out, President Zelensky made a video appearance at Qatar's Doha Forum and raised the possibility of the usage of tactical nuclear weapons in Ukraine. 

"Russia is deliberating bragging they can destroy with nuclear weapons, not only a certain country but the entire planet," he said.

Though there is no international consensus on what tactical nukes are, they have a yield that varies from less than a kiloton to 100 kilotons, and they are smaller than the warheads mounted on Intercontinental Ballistic Missiles, which allows them to be dropped from planes, fitted to short-range missiles, or fired from artillery pieces.

The nuclear bombs used by the United States on Japan during World War II, Fat Man, dropped on Nagaski, and Little Boy, dropped on Hiroshima, had a yield of 21 kilotons and 16 kilotons, respectively.

Russia's nuclear arsenal exceeds that of the US, having around 700 more nuclear warheads than Washington. Moscow has repeatedly underlined its usage of nuclear weapons as a means of deterrence, stressing that it could use nuclear weapons in the event of a first nuclear strike on its territory or infrastructure or if the Russian state as a whole was facing an existential threat posed by nuclear or conventional weapons.

The US recently highlighted that it had a similar stance to Russia's, with the Pentagon saying Washington's nukes are intended to serve as a deterrent to nuclear attacks on the US and its allies, though they might also be considered as an option in "extreme circumstances to defend the vital interests of the United States or its allies and partners."

At the outset of April, the United States military canceled a test of its Minuteman III intercontinental ballistic missile, which it had planned only to postpone in order to reduce nuclear tensions with Russia amid the Ukraine conflict.

The Pentagon first announced a postponement of the test on March 2 after Russia said it will place its nuclear forces on high alert. Washington stated at the time that it was critical for both the US and Russia to "bear in mind the risk of miscalculation and take steps to reduce those risks."