Military setbacks and "potential desperation" could prompt Russian President Vladimir Putin to use "tactical" or "low-yield" nuclear weapons against Ukraine, CIA Director William Burns warned during a speech at Georgia Tech on Thursday.

Why it matters: So-called "tactical" nuclear weapons are considered "low-yield" only because other nuclear weapons have become unimaginably powerful. Any nuclear strike against Ukraine would be far more powerful and devastating than any conventional attack.

• Burns stressed that the intelligence community has seen no "practical evidence" that would suggest such a nuclear attack against Ukraine was imminent but it was not ignoring the threat.

What they're saying: Given "the potential desperation of President Putin and the Russian leadership, given the setbacks that they've faced so far militarily, none of us can take lightly the threat posed by a potential resort to tactical nuclear weapons or low-yield nuclear weapons," Burns said.

• While "we've seen some rhetorical posturing on the part of the Kremlin, about moving to higher nuclear alert levels, so far we haven't seen a lot of practical evidence of the kind of deployments or military dispositions that would reinforce that concern," he added.
• "We watch for that very intently. It's one of our most important responsibilities at CIA."

How it works: Russia has in its nuclear arsenal thousands of smaller warheads, and multiple different systems that can deliver those warheads in shorter and intermediate ranges.

• The explosive power of "low-yield" nuclear bombs can range from 50 kilotons (one kiloton is equivalent to 1,000 tons of TNT) to 20, 10 or .1 kilotons.
• "Little Boy," the nuclear weapon the U.S. dropped on Hiroshima on Aug. 6, 1945, and the first weapon of its kind used in combat, had a yield of 15 kilotons and killed an estimated 66,000 people and injured 69,000 more.
• Tactical nuclear weapons have never been governed by a formal nuclear arms control treaty between the U.S. and Russia.

The big picture: Shortly after the start of Russia's invasion of Ukraine, Putin placed the country's nuclear deterrent forces on alert.

• Kremlin officials have repeatedly warned that Russia would use nuclear weapons if there's a "threat for existence" to the country.
• Burns said that Russian military doctrine allows for the use of such weapons in order to de-escalate a conventional military threat.

Go deeper: Russia threatens to deploy nukes to Baltic if Sweden and Finland join NATO