US President Joe Biden said Tuesday there was still time to avert the "worst case scenario" of a bloody full-scale Russian invasion of Ukraine through diplomacy.

"There's no question that Russia is the aggressor, so we're clear eyed about the challenges we're facing," Biden said in a nationwide address from the White House.

"Nonetheless, there is still time to avert the worst case scenario that will bring untold suffering to millions of people if they move as suggested."