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 planning to enter Ukraine.
However, Moscow has deployed an estimated 100,000 soldiers near the border.
The Kremlin has said it sees the Western military alliance Nato as a security threat and is demanding legal guarantees that it will not add new members further east, including neighbouring Ukraine. But the US has said the issue at stake is Russian aggression, not Nato expansion.
Fears of invasion have prompted Western embassies in Kyiv to withdraw some personnel.