Monitor and advise...something USA is good at in all wars in the Middle East, as they won all the battles there.
No nation is stupid enough to launch a nuclear strike cuz some foot soldiers got into skirmish.
Russia would already be knowing, USA has its proxies in place who know the georgraphy and demography better than US special forces monitoring and advising Ukranian force.
Threshold for nuclear first strike for Russia would be when USA nuke putins Gold Toilet.
Even I would be angary if someone does that to my loo.