US president Joe Biden has said that any Russian troop movements across Ukraine’s border would constitute an invasion, saying that Moscow would “pay a heavy price” for such an action.

His stark warning to Russian president Vladimir Putin came in remarks from the White House and was another effort to clear up any confusion about the position of the US and its Nato allies.

Mr Biden was heavily criticised on Wednesday for saying a “minor incursion” by Russia would elicit a lesser response.

“I’ve been absolutely clear with president Putin. He has no misunderstanding – any, any assembled Russian units move across the Ukrainian border, that is an invasion,” Mr Biden said.

“Let there be no doubt at all, if Putin makes this choice, Russia will pay a heavy price.”

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A Ukrainian soldier in the trench on the line of separation from pro-Russian rebels, Mariupol, Donetsk region, Ukraine (Andriy Dubchak/AP)

His comments came as US secretary of state Antony Blinken prepares to meet Russian foreign minister Sergey Lavrov on Friday in Geneva, in a high-stakes bid to ease tensions that appears likely to fail.

On Wednesday, Mr Biden said he thought Moscow would invade and warned Mr Putin that his country would pay a “dear price” in lives lost and a possible cut-off from the global banking system if it did.

But he also prompted consternation among allies after saying the response to a Russian invasion “depends on what it does”.

“It’s one thing if it’s a minor incursion and then we end up having a fight about what to do and not do, et cetera,” he said.

On Thursday Mr Biden said that “Russia has a long history of using measures other than overt military action to carry out aggression – paramilitary tactics, so-called grey zone attacks and actions by Russian soldiers not wearing Russian uniforms.”

Ukrainian president Volodymyr Zelenskyy was among those expressing concern about Biden’s “minor incursion” remark.

“We want to remind the great powers that there are no minor incursions and small nations. Just as there are no minor casualties and little grief from the loss of loved ones,” he tweeted.

Earlier, Mr Blinken warned in Berlin at a news conference with his German counterpart: “If any Russian military forces move across the Ukrainian border and commit new acts of aggression against Ukraine, that will be met with a swift, severe, united response from the United States and our allies and partners.”

Later, he accused Russia of threatening the foundations of world order with its build-up of an estimated 100,000 troops near Ukraine. He said Russia must face a concerted and severe global response if it invades.

He said Russia’s actions toward Ukraine are an attempt to subvert international norms and just the latest in a string of Moscow’s violations of numerous treaties, agreements and other commitments it has made to respect the sovereignty and territory of other countries.

“To allow Russia to violate those principles with impunity would drag us all back to a much more dangerous and unstable time, when this continent, and this city, were split in two, separated by no-man’s lands patrolled by soldiers, with the threat of all-out war hanging heavily over everyone’s lives,” Mr Blinken told an audience at the Berlin-Brandenburg Academy of Sciences.

“It would also send a message to others around the world that these principles are expendable.”

“We will not treat the principles of sovereignty or territorial integrity as negotiable,” he said, adding that the situation is “bigger than a conflict between two countries, and it’s bigger than a clash between Russia and Nato.

“It’s a crisis with global consequences and it requires global attention and action.”

Mr Blinken took pains on Thursday to stress the US and its partners were united, noting that American diplomats have held more than 100 meetings with allies in recent weeks “to ensure that we are speaking and acting together with one voice when it comes to Russia”.

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Ukrainian soldiers on the line of separation from pro-Russian rebels, in Mariupol (Andriy Dubchak/AP)

Russia denies it is planning an invasion and, in turn, accused the West of plotting “provocations” in Ukraine, citing the delivery of weapons to the country by British military transports in recent days.

Russian foreign ministry spokeswoman Maria Zakharova alleged Ukrainian and Western talk of an imminent Russian attack was a “cover for staging large-scale provocations of their own, including those of military character”.

Russia wants binding security guarantees, including a permanent prohibition on Ukrainian membership in Nato, to which Kyiv aspires, and the removal of most of the US and allied military presence in eastern Europe.

Russia on Thursday announced sweeping naval manoeuvres throughout February, some apparently in the Black Sea, involving more than 140 warships and more than 60 aircraft.