CIA Director William Burns said Thursday that Russia’s invasion of Ukraine may force President Vladimir Putin to use a tactical or low-yield nuclear weapon.

“None of us can take lightly the threat posed by a potential resort to tactical nuclear weapons or low-yield nuclear weapons,” Burns said during a speech in Atlanta, “given the potential desperation of President Putin and the Russian leadership, given the setbacks that they’ve faced so far, militarily.”

The Kremlin said it put Russian nuclear forces on high alert shortly after the assault began on February 24, but Burns told students at Georgia Tech that the US has not seen “a lot of practical evidence” of actual deployments that would cause more concern.

“We’re obviously very concerned. I know President Biden is deeply concerned about avoiding a third world war, about avoiding a threshold in which, you know, nuclear conflict becomes possible,” said Burns.

Russia has many tactical nuclear weapons, which are less powerful than the bomb the United States dropped on Hiroshima during World War II.

Russian military doctrine features a principle called escalate to de-escalate, which would involve launching a first strike nuclear weapon of low yield to regain the initiative if things go badly in a conventional conflict with the West.

But under this hypothesis, “NATO would intervene militarily on the ground in Ukraine in the course of this conflict, and that’s not something, as President Biden has made very clear, that’s in the cards.”

Recalling that he once served as US ambassador to Russia, Burns had very harsh words for Putin, calling him an “apostle of payback” who over the years “has stood in a combustible combination of grievance and ambition and insecurity.”