US Secretary of State Antony Blinken has warned that there would be a "swift, severe" response from the United States and its allies if Russia sends any military forces into Ukraine.

Blinken's comments in Berlin on Thursday appeared to be another effort to clear up any confusion about the position of the US and its Nato allies after President Joe Biden was heavily criticized for saying a "minor incursion" by Russia would elicit a lesser response.

"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," Blinken told a news conference with his German counterpart.

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

The stark warning was delivered in Berlin, the formerly divided city that symbolised the Cold War split between East and West, as Blinken prepares to meet on Friday in Geneva with Foreign Minister Sergey Lavrov in a high-stakes bid to ease tensions that appears likely to fail.

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

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," 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."

The speech came after Blinken and top diplomats from Britain, France and Germany met in Berlin to project a united front to Russia over concerns that it may be planning to invade Ukraine. A day earlier, he met Ukraine's president in Kiev to discuss the threat.

Biden said on Wednesday that he thinks Moscow will invade and warned Russian President Vladimir Putin that his country would pay a "dear price" in lives lost and a possible cut-off from the global banking system if it does.

But, Biden also prompted consternation among allies after saying that 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 said on Twitter.

Blinken took pains on Thursday to stress that the US and its partners were united in the face of Moscow's actions, 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".

"That unity gives us strength, a strength I might add that Russia does not and cannot match," he said. "It's why we build voluntary alliances and partnerships in the first place. It's also why Russia recklessly seeks to divide us."

Russia has denied it is planning an invasion and, in turn, accused the West on Thursday of plotting "provocations" in Ukraine, citing the delivery of weapons to the country by British military transport planes in recent days.