In a Sunday phone call Joe Biden told Ukraine's President Volodymyr Zelensky that the US and its allies will "respond decisively if Russia further invades Ukraine." Crucially, Biden pledged that the US would not go over Ukraine's head in reaching any final resolutions with Russia during upcoming negotiations and talks over the crisis. Biden vowed to Zelensky that key to US-Ukraine relations would being "nothing about you without you" would be done.
Ukraine has in the past months on numerous occasions expressed fear of such a scenario as the White House engaging Putin but without the direct input of Kiev. Zelensky himself has echoed the concern of being "sidelined" on issues directly pertaining to Ukraine's security.
But as with prior instances of Biden phone calls and summits with Putin, Zelensky's call came days after the Thursday one-hour phone dialogue with Putin. Typically Ukraine is consulted after the two superpowers engage, in a pattern that has frustrated Kiev, also at a moment it believes Russia's military is preparing for an offensive into Donbass by month's end.
Biden further pledged to look out for Ukraine's "sovereignty and territorial integrity" and "He also expressed support for confidence-building measures to de-escalate tensions in Donbas and active diplomacy to advance the implementation of the Minsk Agreements, in support of the Normandy Format," according to the White House call read-out.
Special talks in Geneva where the Russian, US and NATO sides are expected to engage on Moscow's submitted "security guarantees" draft document will kick off Jan.9-10. Central to Russia's position is that it receive legally binding commitments of no further NATO expansion eastward. Further it wants to see prior historic verbal pledges given to Georgia and Ukraine of a "path to NATO membership" ultimately rescinded.
Zelensky for his part, issued a message on Twitter on Sunday evening, describing the Biden call as affirming "the special nature of our relations." He added that "Joint actions of Ukraine, United States and partners in keeping peace in Europe, preventing further escalation, reforms, deoligarchization were discussed."
In terms of specific warnings that Biden delivered to Putin last week, Russia was warned of a "heavy price to pay" if there's any offensive into Ukraine, which so far has included talk of "heavy economic sanctions" - which likely means Iran-style sanctions involving export controls.