Biden said a third summit with Putin “is still a possibility” after the two leaders met twice last year. He said he was concerned that a Ukraine conflict could have broader implications and “could get out of hand”.

Speaking to reporters at length about the crisis threatening to engulf his presidency, Biden said he believed Putin would test Western leaders. The response to any Russian invasion, he said, would depend on the scale of Moscow’s actions and whether US allies squabbled over how to react.

“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 … it is going to be a disaster for Russia if they further invade Ukraine,” Biden added. Biden and his team have prepared a broad set of sanctions and other economic penalties to impose on Russia in the event of an invasion.

Biden, pressed on what he meant by a “minor incursion”, said Nato allies are not united on how to respond depending on what exactly Putin does, saying “there are differences” among them and that he was trying to make sure that “everybody’s on the same page”.

Russia moves troops west, US warns Ukraine could be attacked ‘at any point’

“Big nations can’t bluff, number one. Number two, the idea that we would do anything to split Nato … would be a big mistake. So the question is, if it’s something significantly short of a significant invasion or … just major military forces coming across.

For example, it’s one thing to determine if they continue to use cyber efforts; well, we can respond the same way,” he said.

US officials view limiting Nato expansion as a non-starter, but Biden suggested there could be a deal under which the West might not station nuclear forces in Ukraine.

Visiting Kyiv in a show of support, US Secretary of State Antony Blinken said Russia could launch a new attack on Ukraine at “very short notice” but Washington would pursue diplomacy as long as it could, even though it was unsure what Moscow really wanted.

The Kremlin said tension around Ukraine was increasing and it still awaited a written US response to its sweeping demands for security guarantees from the West, including a halt to further Nato expansion and a withdrawal of alliance forces from central and eastern European nations that joined it after 1997.

The pessimistic statements highlighted the US-Russian gulf ahead of talks between Blinken and Russian Foreign Minister Sergei Lavrov on Friday that one Russian foreign policy analyst called “probably the last stop before the train wreck”.

Blinken said he would not present a written response on Russia’s proposals to Lavrov in Geneva but promised “relentless diplomatic efforts to prevent renewed aggression and to promote dialogue and peace”.

He said a Russian build-up of tens of thousands of troops near Ukraine’s borders was taking place with “no provocation, no reason”.

UK supplies weapons, Canada deploys special forces to Ukraine

“We know that there are plans in place to increase that force even more on very short notice, and that gives President Putin the capacity, also on very short notice, to take further aggressive action against Ukraine,” he said.

Russia has also moved troops to Belarus for what it calls joint military exercises, giving it the option of attacking neighbouring Ukraine from the north, east and south.

Eight years ago it seized Crimea and backed separatist forces who took control of large parts of eastern Ukraine, but it has consistently denied any intention of invading now.

Kremlin spokesman Dmitry Peskov said Western weapons deliveries to Ukraine, military manoeuvres and Nato aircraft flights were to blame for rising tensions around Ukraine.