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.
