Ukraine's allies insist they will be unified in responding strongly to any Russian assault, shifting into damage control after US President Joe Biden suggested divisions about how to react to a "minor incursion".
Russia has massed tens of thousands of troops on its borders with Ukraine and some governments fear officials in Moscow are planning a new assault on a country Russia invaded in 2014.
Russia denies it is planning an attack but says it could take unspecified military action if a list of demands are not met, including a promise from NATO never to admit Ukraine as a member.
At a news conference on Wednesday, Biden said he expected Russian President Vladimir Putin to launch some kind of action and appeared to suggest the US and its allies could argue over the response if Russia stopped short of a major invasion.
"My guess is he will move in," Biden said.
"He has to do something."
"Russia will be held accountable if it invades - and it depends on what it does," Biden said.
"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, adding that an invasion would be a "disaster" for Russia.
Shortly after the nearly two-hour news conference ended, the White House rowed back from any suggestion that a smaller-scale Russian military incursion would meet a weaker US response.
"If any Russian military forces move across the Ukrainian border, that's a renewed invasion, and it will be met with a swift, severe and united response from the United States and our allies," White House press secretary Jen Psaki said.
European allies made similar remarks stressing a unified position and threatening strong financial measures against Russia for any assault.
Europe would respond to a new attack "with massive economic and financial sanctions. The transatlantic community stands firm in this," European Commission President Ursula von der Leyen said.
UK Prime Minister Boris Johnson said: "Be in no doubt that if Russia were to make any kind of incursion into Ukraine, or on any scale, whatever, I think that that would be a disaster, not just for Ukraine, but for Russia. It would be a disaster for the world."
US Secretary of State Antony Blinken was in Germany on Thursday, meeting French, British and German ministers ahead of talks on Friday with Russian Foreign Minister Sergei Lavrov.
With European countries having long emphasised their united position in public, some officials privately expressed frustration at Biden's remarks although they described it as a gaffe that was unlikely to alter Russia's calculations.
"It was not helpful, in fact it was a gift to Putin, but we should not read too much into it. Biden has not given Moscow the green light for an attack on Ukraine. It was a slip of his tongue, and the official Western position will prevail," one security source said.
Another said: "I think the Russians will know how to rank Biden's remarks, they will classify it as what it was - a gaffe."
Ukraine's President Volodymyr Zelenskiy on Thursday said there were no "minor incursions and small nations" in an unusually blunt riposte to Biden's comments.
"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. I say this as the president of a great power Ukraine," he tweeted.
Australian Associated Press
