Two of the people said about half that number of tactical groups was already in position and that any invasion would be backed up by air support. 

The two people said that Moscow had also called up tens of thousands of reservists on a scale unprecedented in post-Soviet times. They explained the role of reservists in any conflict would be to secure territory in a later phase after the tactical battalions paved the way. Russia has not publicly announced any major call-up of reservists.

One of the people said the US had also shared information about an exponential rise in disinformation targeting Kyiv and that Moscow has recruited agents to try and sow destabilisation inside Ukraine.

Putin last week denied any intention to invade but welcomed the alarm as evidence that his actions had got the attention of the US and its allies, which he accused of failing to take Russia’s “red lines” over Ukraine seriously enough.

A senior administration official said the US has under Biden showed it is willing to use a number of tools to address harmful Russian actions and would continue to do so. The White House said it had no further comment.

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The Russian leader has a history of brinkmanship and prior build-ups on the border have come to naught. But officials say he is positioning to act if he wanted to, and they note that even as troops withdrew from an earlier escalation in April they left equipment behind, making the fresh build-up faster and easier.

For Putin, Ukraine is unfinished business after his annexation of Crimea in 2014. He sees Ukraine as part of Russia and bristles at its outreach with the west, especially its nascent military engagement with Nato.

While he has said he does not want a war, in a speech last week he said his goal was to keep the US and its allies on edge for “as long as possible, so nobody gets it into their head to cause a conflict we don’t need on our western borders”.

He lambasted the US and others for expanding military infrastructure in Ukraine and stepping up naval missions in the Black Sea and flights by warplanes along Russia’s borders.

At the same time, Russian authorities understand that any attempt to occupy large amounts of Ukrainian territory militarily would face widespread public opposition on the ground and trigger sweeping western sanctions that could batter the Russian economy, people close to the leadership said.

Further diplomatic flurries are likely, while the US has been considering re-upping and expanding a package of measures it put together this year during the prior troop build-up which could include sanctions.

Ukrainian Defence Minister Oleksii Reznikov visited Washington last week and asked the Pentagon for more help defending the country’s airspace and coast. Ukraine’s foreign minister also travelled to Brussels last week.

Echoing the US concerns, its defence intelligence agency chief told the Military Times he believed Russia could be preparing for an attack by the end of January or early February involving air strikes, amphibious assaults and an incursion though Belarus.

Kremlin spokesman Dmitry Peskov dismissed such talk on Sunday, saying, “this hysteria is being built up artificially”.

“The ones who are accusing us of some kind of unusual military activity on our own territory are themselves sending their armed forces from across the ocean. I mean the United States of America. It’s not very logical and not very decent,” he told state television. 

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For Biden, the actions of the Russian president present multiple challenges. They could freshly test his administration’s appetite to intervene militarily offshore. America only months ago had a messy withdrawal of its troops from Afghanistan after two decades of conflict there. Biden has had to focus political strength on domestic priorities like his economic agenda and dealing with rising prices at the gas pumps and grocery store registers.

The latest frictions come against a broader backdrop. Not only are troops massing again, but record-high gas prices leave Europe vulnerable to Russian largesse on supplies. European Union members including Poland have also accused Russia of encouraging a refugee crisis as its ally Belarus funnels migrants toward the bloc. And a Russian weapons test that blasted a satellite out of orbit scattered a large field of debris, prompting US accusations it had endangered the International Space Station’s crew. 

Still, one of the people with knowledge of US intelligence said these events are not believed to be part of a wider plan but are being exploited opportunistically by the Russian president.

Tensions between Russia and Ukraine blew up in 2014, when Moscow annexed the Crimean Peninsula and backed a separatist rebellion in the east of the country. That triggered sweeping US and European sanctions, but the conflict has continued, killing about 14,000 people in east Ukraine. Two of the people said that any new attack would be on a scale far greater than seven years ago.

One of the people said a potential joint response needed to be agreed soon among allies that would be unequivocal and overwhelming if implemented, adding there was still a window of opportunity to deter Putin. It’s unclear at this stage if that would include a possible military response. French Foreign Minister Jean-Yves Le Drian on Sunday said any violation of the Ukraine border by Russia would lead to “extremely grave consequences.”

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The US warnings of potential invasion are seen in Moscow as a sign that Washington – at least so far – has no intention of compromise and is rallying transatlantic unity against Russia.

“The US is laying the groundwork for a unified sanctions response,” said Dmitry Suslov, an expert on Russia’s relations with the US and Europe at the Higher School of Economics in Moscow. 

While Western nations do not currently know the Kremlin’s intentions, they believe that military options are being presented and have little doubt that what they’re hearing and seeing would allow for a large scale attack should Putin decide to make a move in the new year. The people said the US had high confidence in the reliability of the information it had shared with allies.