US President, Joe Biden says he will 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.
Taking questions from reporters, 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.
Mr Biden added that he would feel obliged to beef up Nato’s presence in eastern Europe.
“We have to make it clear that there’s no reason for anyone, any member of Nato, to worry whether… Nato would come to their defence,” he said but he repeated that there were no plans to send US troops to Ukraine itself.
Russia responded angrily to the remarks and accused the US and NATO of “flooding” Ukraine with weapons and Western advisers.
“There is no explanation for what the American fleet is doing near the Russian coast,” Moscow’s Permanent Mission to the United Nations said in a statement.
Russia currently provides about one-third of the crude oil and gas imported by the European Union.
Earlier UK Prime Minister Boris Johnson said the Western allies would respond to any incursion with “severe” economic sanctions, adding that Britain was prepared to deploy troops to protect Nato allies in the region.