The US leader said he has been "absolutely clear with President Putin" that "if any assembled Russian units move across the Ukrainian border, that is an invasion".

That invasion, he said, would be met with a "severe and coordinated economic response," which has been "laid out very clearly with President Putin".

It comes as Liz Truss also warned Russia that an invasion of Ukraine would lead to a "terrible quagmire and loss of life" similar to the Soviet-Afghan war.

Speaking in Sydney, Australia, the British foreign secretary urged Mr Putin to "desist and step back from Ukraine before he makes a massive strategic mistake", adding an "invasion will only lead to a terrible quagmire and loss of life, as we know from the Soviet-Afghan war and conflict in Chechnya".

"We need everyone to step up. Together with our allies, we will continue to stand with Ukraine and urge Russia to de-escalate and engage in meaningful discussions. What happens in eastern Europe matters for the world," she added.

Mr Biden also acknowledged that Russia could "use measures other than overt military actions," and referenced the incursion into the Donbas region, where "grey zone attacks" were undertaken by "Russian soldiers not wearing Russian uniform".

The US president appeared to be clarifying the confusion around the position of America and it's NATO allies after he was heavily criticised for saying a "minor incursion" would elicit a lesser response.

The US Treasury Department has also levied new sanctions against four Ukrainian officials, including two current members of parliament, who they say are part of the Russian influence effort to set the pretext for an invasion.

The sanctions name parliamentarians Taras Kozak and Oleh Voloshyn and two former government officials.

According to the government, all four have been involved in disinformation efforts by Russia's federal security service, the FSB.

US Secretary of State Antony Blinken is preparing to meet his Russian counterpart, foreign minister Sergei Lavrov, in Geneva today, in a bid to ease tensions between the country and the West.

However, talks seem likely to fail and the US diplomat said: "These are difficult issues we are facing and resolving them won't happen quickly.

"I certainly don't expect we'll solve them in Geneva tomorrow."

Meanwhile, Mr Biden appeared convinced Russia would mount an attack, and said: "My guess is he will move in; he has to do something."

However, he warned that Mr Putin would pay a "dear price" in lives lost and a possible cut off from the global banking system if he does.

'Russia looking for excuse to invade Ukraine' - but US insists week of diplomacy has not failed

Military drills aimed at protecting "Russian national interests"

On Thursday, Russia announced sweeping naval drills in several parts of the world and claims the West is plotting "provocations" in neighbouring Ukraine.

An estimated 100,000 Russian troops are now believed to be on the border with Ukraine as part of massive joint war games with Belarus, which will involve more than 140 warships and 60 aircraft.

"The drills are intended to practice navy and air force action to protect Russian national interests in the world's oceans and to counter military threats to the Russian Federation," the Russian ministry said.

In response, Britain's armed forces have flown thousands of anti-tank weapons to the former Soviet satellite state amid fears of an imminent invasion of Ukraine.

Defence Secretary Ben Wallace, who is also with Ms Truss in Australia, added that globalisation has meant that "nations can interrupt and corrupt our democratic and free and open societies from as far away as countries such as Russia and therefore we have to work together to strengthen those alliances".

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Russia has repeatedly denied it plans to launch an offensive but has sought reassure from the West that NATO's expansion, and deployment of alliance weapons, will exclude Ukraine and other ex-Soviet nations.

Ukraine-Russia tensions: Inside the eerie village on the frontline of separatist conflict

Washington and its allies firmly rejected these demands in security talks last week, but have kept the door open to possible further talks on arms control and confidence-building measures to reduce tension in the region.