It’s a troubling question with no palatable answer: What would President Joe Biden do if Russia used nuclear weapons in the Ukraine war?

A half dozen current and former government officials briefed on the issue, and several outside experts, told NBC News there was no playbook and little agreement about how the U.S. would respond to a norm-shattering act of destruction that could obliterate a Ukrainian city, kill tens of thousands and send a cloud of nuclear fallout drifting over NATO countries in Western Europe.

This isn’t new to the Biden administration. In fact, when the Obama administration conducted a war game simulating Russian use of nuclear weapons in the Baltics, there were fundamental disagreements about how to react.

U.S. intelligence officials say they have seen no signs that Russian President Vladimir Putin is poised to employ so-called battlefield nukes, but several versions of Russian military doctrine published since 2000 have envisioned the first use of nuclear weapons in response to a conventional threat in a regional war in response to a conventional threat against the Russian homeland. And military experts say Russia’s smallest warheads have many times the explosive power of the bombs dropped on Hiroshima and Nagasaki.

As the U.S. continues to send ever more sophisticated weapons designed to help Ukraine destroy invading Russian forces, American officials tell NBC News the Biden administration has for months been thinking the unthinkable about what Putin could do — and war-gaming scenarios envisioning Russia using an atomic bomb on Ukraine.

“We don’t see … practical evidence at this point of Russian planning for the deployment or even potential use of tactical nuclear weapons,” CIA Director William Burns said last month. But, he added, “given the kind of saber-rattling that … we’ve heard from the Russian leadership, we can’t take lightly those possibilities.”

Read more at NBC News