US President Joe Biden says he would consider personal sanctions on Vladimir Putin if Russia invades Ukraine.
Mr Biden said there would be “enormous consequences” for the world if Russia made a move on the nation, which sits on its south-western border.
His comments came as other Western leaders repeated warnings that Russia would pay a heavy price for invasion.
Russia has accused the US and others of “escalating tensions” over the issue and denies it plans to enter Ukraine.
However, Moscow has built up troops at the border, with some 100,000 Russian soldiers deployed in the region.
Taking questions from reporters, Mr Biden replied “yes” when asked whether he could see himself imposing sanctions on the Russian president personally the event of an invasion.
He said such a move across Ukraine’s border would mean “enormous consequences worldwide” and could amount to “the largest invasion since World War Two”.
During crisis talks on Monday, Western powers agreed to “unprecedented” sanctions against Russia if it were to invade.
The US has also put 8,500 troops on alert – partly to help reinforce Nato allies – which Russia said caused it “great concern”.
The Kremlin has said it sees Nato as a security threat, and is demanding legal guarantees that the alliance will not expand further east, including into neighbouring Ukraine. But the US has said the issue at stake is Russian aggression, not Nato expansion.
Russia has seized Ukrainian territory before, when it annexed Crimea in 2014. After Russian forces seized control, Crimea voted to join Russia in a referendum the West and Ukraine deemed illegal.
Russian-backed rebels also control areas of eastern Ukraine near Russia’s borders. That conflict has cost an estimated 14,000 lives, with a 2015 peace deal a long way from being fulfilled.
Tensions over Ukraine
- EXPLAINED: Is Russia preparing to invade Ukraine?
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- UK SUPPORT: UK says it is sending weapons to defend Ukraine