The Ukrainians are warning that Russian nuclear threat is rising. The Americans are asking India and China to intercede. This is not good.
I've said from day one that this exact scenario that Russia would consider the use of nuclear weapons.
No they don't need a pretext, and yes any pretext would be beyond flimsy, but dictators love making it look like there brutality and horrible decisions are justified. It's domestic politics 101.
It's also why the annexation of Ukrainian territory is not insignificant. No it doesn't change the situation on the battlefield, but it does change the Russian calculus. Look at all the pomp and ceremonies they're preparing in Moscow for this event. It is meant to be a considerable achievement and change to the domestic sphere, it's meant to reframe the conflict, and open up numerous options that previously "weren't available" to the Kremlin.
Despite the US saying they see no change in the overall risk, their rhetoric suggests otherwise. The fact that they've been more vocal about how they have articulated to the Kremlin what sort of responses a nuclear strike would entail, and more publicly warning against it suggests that they do in fact believe that risk is now elevated.
That said, I don't think Putin is ready to use nukes just yet. If he can stabilize the front that risk subsides for now. Should he fail to stabilize the front in the coming months alongside his mobilization (which unfortunately I believe is highly likely) then that threat will be quite high.
There should be no mistake, Putin knows he will be threatened if he uses nukes, but if he is feeling threatened and like he's on the verge of losing anyway, it's a gamble there's very little reason for him not to make.
Though again, we are not at this point YET.