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. Ukraine's foreign minister Dmytro Kuleba however told reporters on Wednesday that while the troop build up "poses a threat to Ukraine", he believes it is "insufficient for a full-scale offensive".
Diplomats from Russia, Ukraine, Germany and France gathered in Paris on Wednesday for talks about the ongoing tensions.
'Nothing ruled out'
Taking questions from reporters on Tuesday, Mr Biden replied "yes" when asked whether he could see himself imposing sanctions on the Russian president personally in the event of an invasion.
Such a move across Ukraine's border would mean "enormous consequences worldwide" and could amount to "the largest invasion since World War Two", he said.
On Wednesday UK Foreign Minister Liz Truss echoed Mr Biden's words, telling the BBC's Today programme that the government has "ruled nothing out" - including sanctions against the Russian leader - and said the UK is looking to "toughen up" its sanctions on people, banks and businesses.
"What is important is that all of our allies do the same. It is by collective action. By showing Putin we're united, we will help deter a Russian incursion," she said.
Western powers agreed to "unprecedented" sanctions against Russia if it were to invade during crisis talks earlier this week.
On Tuesday UK Prime Minister Boris Johnson raised the issue of banning Russia from the Swift international payments system, a move which senior Russian officials said meant Europe would not be able to pay for and receive Russian products.
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Russia responded angrily to Mr Biden's remarks. Dmitry Peskov, spokesman for Mr Putin, said such sanctions would not be "painful" for the president, but would be "politically destructive".
He added that Russia's top government officials were banned from having any assets or accounts abroad, and that Mr Putin's salary went to the Rossiya bank.
Separately, Mr Biden's administration said it was working with oil and gas suppliers around the world to boost shipments to Europe in the event of Russia cutting off supplies, the New York Times reports.
Russia currently provides about one-third of the crude oil and gas imported by the European Union.