US President Joe Biden demanded a de-escalation of tensions with Ukraine in a phone call with Russian President Vladimir Putin on Thursday.
"He made clear that the United States and its allies and partners will respond decisively if Russia further invades Ukraine," according to a White House statement.
Any Russian military interventions in Ukraine would lead to tough sanctions for Moscow and an expansion of NATO's presence in its eastern member states, a senior White House official said after the talks.
Ukraine could then also count on additional aid, including from its armed forces and for national defence.
In the call that lasted barely an hour, Biden also pushed for a diplomatic solution and expressed support for talks at NATO and the OSCE due early next year.
The call came ahead of talks in Geneva on 10 January, during which diplomats from both countries will meet to discuss the stand-off.
The Russian side expressed satisfaction with the call, with Putin's foreign policy adviser Yuri Ushakov saying if negotiations are successful it could lead to an improvement in bilateral relations.
Putin appeared ready to make concessions to his demands for binding security guarantees from NATO, outlined in a draft agreement submitted two weeks ago.
It called for an end to NATO's eastward expansion, which Moscow sees as a threat.
Ushakov said late Thursday Moscow would "take into account considerations of the US side and our partners in Western countries in negotiations," in comments reported by Interfax.
He said Russia would however insist on guarantees for its own security and that Putin had discussed this with Biden.
Russian troops
The US accuses Russia of a massive build-up of troops near Ukraine, while Russia accuses Ukraine of bulking up its military presence in the region.
The Kremlin did not immediately comment on the warning of sanctions.
For his part, Putin struck a more conciliatory tone in a Christmas and New Year message to Biden on Thursday.
Both countries bear "special responsibility for international and regional stability," Putin said, according to a statement from the Kremlin.
Russia and the US "can and must cooperate constructively and pool efforts in the face of numerous challenges and threats humanity is faced with," the statement said.
Biden and Putin previously spoke in early December during a videoconference that lasted about two hours. They met for the first time in person as heads of state in Geneva in June.
Direct military intervention by the US or NATO is considered unlikely in the event of a Russian invasion of Ukraine.
Any response from Washington and its European allies would be expected to take the form of drastic economic sanctions. Among other things, Russia could be excluded from the international payments system of the Belgium-based organization Swift.
