The US knows more than European allies, hence difference in rhetoric between NATO/US and Germany/France.
During the security council meeting, NATO aligned countries made specific point not to mention Minks agreement, whereas neutral or Russia/China aligned countries made sure to mention Minks agreement. Minks agreement of course boils down to federalisation of Ukraine and perpetual veto on any NATO ascension.
Perhaps Ukraine is building up forces/equipment/expertise(especially with new Turkish drones and domestic production of thereof) to try to retake Donbas. If Ukraine tries to retake Donbas, Russia will not be able to support separatist element as it will not be enough and will have to invade formally (I am sure right now somebody will say it's not separatists it's all Russian forces but during the UN security council meeting Ukrainian representative said Donbas separatist forces stand at 30k with only 10% being Russia military).
So if Ukraine will eventually try to retake Donbas then Russian may have reason to pre-emtively invade to neuter offensive capabilities of Ukraine. The economic loss from sanctions is likely to be immense but if Russia believes that this is inevitable then no reason to delay it.
For me, that's the only scenario in which it makes sense for Russia to invade Ukraine at this point (since frozen conflict already exists to prevent NATO ascension).
If Ukraine will not try to retake Donbas by force then all these theatrics are rather pointless, as even Ukrainian president complained that the rhetoric is affecting the economy, so I am not sure why would the US up the rhetoric so much.