The US takes seriously the reckless rhetoric coming out of Russia. However, the US remains focused on supporting Ukraine for as long as it takes through Putin's war of choice and war of aggression. If he is concerned about escalation, he can pull his troops from Ukraine at any moment and stop invading his neighbor.
This is the second major escalation by Russia. First by deploying North Korean troops in the conflict, now by deploying nuclear-capable weapons on the battlefield. The West must respond to these provocations to show Putin that his actions are unacceptable.
Russia has repeatedly warned the US and UK that the firing of its weapons into Russian territory, which cannot be done without involvement of its specialists, would mark their direct involvement in the conflict. Russia prefers to settle this conflict peacefully, but if provocations continue, it has no choice but to respond.
There is a 10% chance that there will be a US-Russia war before 2050, according to the Metaculus prediction community.