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

Mr Biden’s warning is the latest White House effort to clear up comments the American leader made a day earlier when he suggested that a “minor incursion” by Russia into Ukrainian territory could result in a more measured response by the United States and allies.

Facing an avalanche of criticism from Republicans and Ukrainian officials that Mr Biden’s comments had invited limited military action by Russian president Vladimir Putin, the US leader sought to clarify his remarks at the start of a meeting at the White House focused on domestic policy.

“I’ve been absolutely clear with President Putin,” Mr Biden said.

Ukrainian soldiers
A Ukrainian soldier stands in the trench on the line of separation from pro-Russian rebels, in Mariupol, Donetsk region (AP)

“He has no misunderstanding: Any, any assembled Russian units move across the Ukrainian border, that is an invasion.”

Mr Biden added that an invasion would be met with a “severe and coordinated economic response”.

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

Mr Biden said the US is preparing for Russia to take action that falls outside the parameters of conventional warfare.

“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,” he said.

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

But Mr Biden also prompted consternation among allies by 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.

Ukrainian president Volodymyr Zelenskyy was among those expressing concern.

“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.

Before traveling to Geneva, Mr Blinken warned in Berlin that there would be a “swift, severe” response from the United States and its allies if Russia sent any military forces into Ukraine.

“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,” Mr Blinken told a news conference with his German counterpart, Annalena Baerbock.

Later, Mr Blinken accused Russia of threatening the foundations of world order with its buildup of an estimated 100,000 troops near Ukraine.

Russia must face a concerted and severe global response if it invades, he said.

“These are difficult issues we are facing, and resolving them won’t happen quickly,” Mr Blinken said. “I certainly don’t expect we’ll solve them in Geneva tomorrow.”

He later told an audience at the Berlin-Brandenburg Academy of Sciences that Russia’s actions toward Ukraine are an attempt to subvert international norms and just the latest in a series of violations of numerous treaties, agreements and other commitments Moscow has made to respect the sovereignty and territory of other countries.

Antony Blinken
US secretary of state Antony Blinken (AP)

“Perhaps no place in the world experienced the divisions of the Cold War more than this city,” Blinken said. “Here, President Kennedy declared all free people citizens of Berlin. Here, President Reagan urged Mr Gorbachev to ‘tear down that wall’.

“It seems at times that President Putin wants to return to that era. We hope not.”

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.

The US and its European partners say they are willing to consider certain less-dramatic gestures but that the Russian demands are out of the question, and that Mr Putin knows they are non-starters.

That, Mr Blinken said, is proof of Mr Putin’s ulterior motive.

Maria Zakharova
Russian foreign ministry spokeswoman Maria Zakharova (Russian Foreign Ministry via AP)

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

Separately, Spain’s defence minister said the country was sending two warships to the Black Sea with Nato approval.

Amid concerns that Putin may not be moved by threats of sanctions, Mr Blinken made a direct appeal to the Russian people to oppose any intervention.

“You deserve to live with security and dignity, like all people everywhere, and no-one – not Ukraine, not the United States, not the countries of Nato – is seeking to jeopardise that.

“But what really risks your security is a pointless war with your neighbours in Ukraine, with all the costs that come with it – most of all, for the young people who will risk or even give their lives to it,” he said.

Mr Biden has said he is not planning to send combat troops in the case of a further Russian invasion.

But he could pursue less dramatic yet still risky military options, including supporting a post-invasion Ukrainian resistance.