Speaking at a news conference marking a year in office, the US president suggested there was an inevitability about Russian President Vladimir Putin's actions.

"My guess is he will move in; he has to do something," Joe Biden said.

"Do I think he'll test the West, test the United States and NATO? Yes. As significantly as he can? Yes, I think he will," he said.

"But I think he'll pay a serious and dear price for it that he doesn't think now will cost him. And I think he will regret having done it."

Smaller incursion may not prompt massive sanctions

The comments are his first since last week's marathon round of diplomacy with Russia faltered.

And pressed on whether sanctions will work to deter Russia from invading, the president offered a distinction between an invasion and an incursion, appearing to infer that a smaller incursion would not prompt massive sanctions.

"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 a fight about what to do and not do etc, but if they actually do what they are capable of doing with the forces that are massed on the border, it is going to be a disaster for Russia," he said.

He was offered the chance to clarify whether he was suggesting that an incursion would prompt a lesser Western response.

"So the question is if it's something significantly short of a significant invasion, or not even significant, just major military forces coming across. For example, it's one thing to determine that if they continue to use cyber efforts, well we can respond the same way, with cyber," he said.

'Actions would be met by a reciprocal response'

White House staff later clarified his comments, saying that the president was distinguishing between military and non-military or para-military.

Emily Horne, spokesperson for the White House National Security Council tweeted: "He was referring to the difference between military and non-military / para-military / cyber action by the Russians. Such actions would be met by a reciprocal response, in coordination with Allies and partners."

Biden issues warning

President Biden concluded his Ukraine comments in the news conference with a warning.

"We need to be very careful about how we move forward and make it clear to him that there are prices to pay that could in fact cost his country an awful lot," the president said.

With stark language, he added: "Of course you have to be concerned when you have, you know, a nuclear power invade. If he invades, this hasn't happened since World War Two.

"This'll be the most consequential thing that's happened in the world in terms of war and peace since World War Two."

Equally stark, in a comment that might be lost in one of the longest news conferences in presidential history, Mr Biden questioned whether anyone even within the Kremlin had any idea of Putin's plan.

"There is a question about whether the people [we] are talking to [Russian diplomats] know what he [Putin] is going to do."