EU and Nato allies have swung behind the Biden administration’s assessment that Russia may be poised to invade Ukraine, following unprecedented sharing of US intelligence on Moscow’s military preparations.

Weeks of sustained US diplomatic engagement with European governments, backed by a sharing of intelligence normally reserved for its closest allies, have helped convince some previously sceptical capitals, including Berlin, that the Kremlin could soon order its troops into Ukraine. The effort has galvanised support for the need for robust sanctions threats to deter the Kremlin.

US president Joe Biden will warn Russia’s Vladimir Putin against any invasion in a planned video summit on Tuesday, with the full backing of Nato and the EU for retaliatory measures, European defence and security officials told the FT.

Russia could be planning to invade Ukraine “as soon as early 2022”, a Biden administration official said on Friday, adding that half the military units that would be involved in such an offensive had arrived near Ukraine’s border over the past month.

The US decision to share its intelligence so widely among European states and issue public warnings stems from Washington’s hope that, by cementing western support for sanctions, it would underline to Moscow the costs of any aggression. The details of threatened sanctions and other countermeasures are still under discussion.

Many allies were not convinced that serious things were happening

The Kremlin has consistently denied that it plans to invade Ukraine and has blamed increasing tension on US and Nato support for Kyiv.

US intelligence reports depicting Russian military deployments along the Ukrainian border, evidence of possible attack preparations and analysis of the Kremlin’s perceived intentions were shared bilaterally and collectively with Nato members and through EU diplomatic channels, officials briefed on the documents told the FT.

The quantity of material and detail shared among the other 29 Nato allies was described by one official as “extremely comprehensive”.

The uncharacteristic level of intelligence-sharing was prompted by initial reluctance from some European allies to treat US claims that an invasion was being prepared as credible, four of the officials said.

The disclosure of previously secret details began in early November ahead of a meeting of Nato ministers last week, which was subsequently dominated by discussions about Ukraine. The intelligence helped shift the conversation from whether the warning was correct to how to best deter it.

“Many allies were not convinced that serious things were happening,” said a second official. “We were surprised about this [intelligence] gap — how and why the US were seeing things that we were not seeing.”

“If I have to compare soundbites from before this info and then [at the Nato meeting] in Riga, there was a big shift towards the US version of things,” the official added.

Biden said on Friday that he was preparing a “comprehensive and meaningful set of initiatives” to deter any Russian aggression.

“We’ve been aware of Russia’s actions for a long time and my expectation is we’re going to have a long discussion,” he said of the upcoming call with Putin.

The US says Russia has made preparations to deploy 100 battalion tactical groups totalling an estimated 175,000 military personnel at various strategic locations along the Ukrainian border, backed by 100,000 reservist troops.

Russian troops invaded Georgia in 2008, and invaded and annexed Ukraine’s Crimea peninsula in 2014. Since then, the Kremlin has supported pro-Moscow separatists that have fought a seven-year war against Ukrainian government forces in Donbas, a Ukrainian region on the border with Russia.

In April, Russia rapidly and without prior warning moved 100,000 troops from other parts of the country to its border region with Ukraine, alongside tanks, aircraft, naval forces, field hospitals and electronic warfare equipment, spooking Kyiv and western countries. Some of those troops were eventually returned to their bases.

Some EU states and Nato members that have called for dialogue with Moscow rather than confrontation have cited that de-escalation as evidence that Russia would not embark on a full invasion unless provoked. But the US intelligence on the recent troop deployments has shifted that analysis.

The US and the EU have “the same coherent message . . . to show the price Putin will pay” for any action against Ukraine, a third European official said. “Some European states who were not reading the moves from Putin [in the same way that the US was] in terms of intentions, now are.”