I should have said "discreet". That is, told to them directly first, and without bombast (rather than simply bloviating in public, the way Putin's and Trump's people are wont to do). In any case it's all a matter of public record:

  National security adviser Jake Sullivan told NBC News' "Meet the Press" on Sunday that the consequences "would be catastrophic if Russia went down the dark road of nuclear weapons use."

  Secretary of State Antony Blinken used similar language in an interview with CBS News' "60 Minutes."

  "It's very important that Moscow hear from us and know from us that the consequences would be horrific, and we've made that very clear," he said, adding that the U.S. response would be "catastrophic" without elaborating.

https://www.nbcnews.com/news/world/russia-catastrophic-conse...