The Biden administration refused a demand to permanently ban Ukraine from joining the NATO, and said allied deployments of troops and military equipment in Eastern Europe are nonnegotiable.
The United States has rejected Russia's key demand to bar Ukraine from NATO and said it believed Moscow was ready to invade but offered what it called a new "diplomatic path" out of the crisis.
US Secretary of State Antony Blinken said on Wednesday that he would speak again in the coming days to Russian Foreign Minister Sergey Lavrov.
"It sets out a serious diplomatic path forward should Russia choose it," Blinken told reporters of the US response to any eventuality from Russia, which he said would remain confidential.
He renewed an offer on "reciprocal" measures to address mutual security concerns, including reductions of missiles in Europe and transparency on military drills and Western aid to Ukraine.
But he made clear that the United States would not budge on Russia's core demand that Ukraine never be allowed to join NATO, the US-backed military alliance.
"From our perspective, I can't be more clear - NATO's door is open, remains open, and that is our commitment," Blinken said.
There was no immediate response from Russia but Russian officials have warned that Moscow would quickly take “retaliatory measures” if the US and its allies reject its demands.
One month after Russia put forward sweeping security proposals, having sent tens of thousands of troops to Ukraine's border, the US delivered a coordinated reply with NATO allies saying it was ready for any eventuality.
The United States has warned of severe and swift consequences if Russia invades, including possible personal sanctions on President Vladimir Putin, and NATO has put 8,500 troops on standby.
"While we are hoping for and working for a good solution - de-escalation - we are also prepared for the worst," NATO Secretary General Jens Stoltenberg said.
US President Joe Biden, speaking with European leaders by video-conference on Tuesday, said any Russian military attack on Ukraine would trigger "enormous consequences" and could even "change the world." Kremlin warned that attempts to punish Putin personally would be "destructive."
In another bid to defuse tensions, senior Russian and Ukrainian officials met for eight hours in Paris with representatives of France and Germany.
France said after the so-called Normandy Format talks that the envoys committed to a fragile July 2020 ceasefire in eastern Ukraine between government forces and pro-Moscow separatists.
Dmitry Kozak, the Kremlin deputy chief of staff, said the talks were "not simple" but that another round would take place in two weeks in Berlin.