President Joe Biden, press conference, NATO Headquarters Brussels, Belgium, March 24:

Q:...would the U.S. or NATO respond with military action if [Putin] did use chemical weapons?

THE PRESIDENT: ...we would respond. We would respond if he uses it. The nature of the response would depend on the nature of the use.
...
Q: ...And to clarify, on chemical weapons: Could — if chemical weapons were used in Ukraine, would that trigger a military response from NATO?

THE PRESIDENT: It would re- — it would trigger a response in kind, whether or not — you’re asking whether NATO would cross; we’d make that decision at the time.