United States President Joe Biden said on Wednesday that he thinks Russia will invade Ukraine and warned that it would spell "disaster" for Moscow.

Washington has said an attack could happen early this year, which Russian president Vladimir Putin's administration has denied, while a buildup of more than 100,000 Russian troops is estimated to have amassed along Ukraine's border.

"I'm not so sure he is certain what he's going to do. My guess is he will move in. He has to do something," Biden said at a news conference on the eve of his first anniversary in the White House.

The US president warned that Moscow will be held accountable, although he added that the measures that the US and its allies adopt would depend on the type of military intervention that Russia chooses to carry out.

"Russia will be held accountable if it invades – and it depends on what it does. It's one thing if it's a minor incursion and we end up having to fight about what to do and what to not do, et cetera," Biden said. "But if they actually do what they're capable of doing, with the forces it's amassed on the border, it's going to be a disaster for Russia if they further invade Ukraine."

Pressed on what a "minor incursion" would be, Biden admitted NATO is divided on its response to various scenarios and that he was trying to make sure that "everybody's on the same page."

"There are differences in Nato as to what countries are willing to do, depending on what happens," Biden said. "If there's Russian forces crossing the border … I think that changes everything."