The US has said it would send reinforcements to Nato’s eastern flank in the wake of a Russian invasion of Ukraine, as well as imposing severe new economic measures, in a warning to Moscow on the eve of talks between Joe Biden and Vladimir Putin. Biden will also make clear to Putin that the US will not rule out future Ukrainian membership of Nato, as the Russian leader has demanded, a senior US official said. With an estimated 100,000 Russian troops already gathered within striking distance of the borders, the crisis is at its worst since 2015, when Moscow staged a large-scale incursion into Ukraine, clandestinely sending tanks and artillery to encircle Ukrainian troops and compelling Kyiv to sign a peace agreement in Minsk that has since come close to collapse. The official pointed out in a briefing to reporters before the Biden-Putin video summit that the first Russian military intervention in Ukraine led to more US troops and equipment to be deployed in eastern Europe, and that there would be similar response this time. “It would certainly be the case that if Putin moved in, there would be an increasing request from eastern flank allies, and a positive response from the United States, for additional forces and capabilities and exercises to take place there to ensure the safety and security of our eastern flank allies in the face of that kind of aggression in Ukraine,” the official said, but made clear that Biden would not be threatening a direct US military response. “The United States is not seeking to end up in a circumstance in which the focus on our countermeasures is the direct use of American military force, as opposed to a combination of support for the Ukrainian military, strong economic countermeasures, and a substantial increase in support and capability to our Nato allies to ensure that they remain safe,” the senior official said, adding he would not disclose “what the president is going to say on the question of under what circumstances the US military could get involved”. The official said “substantial economic countermeasures” from the US and Europe could “impose significant and severe economic harm on the Russian economy”. Biden was due to talk to European leaders on Monday in preparation for the Putin summit with the aim of ensuring that he will be presenting a united front. The US secretary of state, Antony Blinken, will also consult the Ukrainian president, Volodymyr Zelenskiy, before the Putin call. Putin has said that he will seek “security guarantees” for Russia, including a ban on Nato enlargement or military support for Ukraine. The White House made clear that would not be up for discussion. “The US has consistently expressed support for the principle that every country has the sovereign right to make its own decisions with respect to its security,” the US official said. “That remains US policy today and will remain US policy in the future. So that much is straightforward and clear.” Russia has virtually frozen direct contacts with the Ukraine government and Zelenskiy, whom it has accused of preparing his own attack against regions of south-east Ukraine controlled by Russian-backed forces. Ukraine has strongly denied the claims. The Ukraine defence minister, Oleksii Reznikov, on Friday told its parliament that Russia had an estimated 94,000 troops near the Ukrainian border and may be preparing an offensive beginning in late January. US officials have given similar estimates of Russian troop numbers and potential timeline for an attack as Putin upped his rhetoric about the west crossing Russia’s “red lines” in terms of providing military support to the Kyiv government. Both western and Russian analysts have said that Moscow is making a credible threat of launching a large-scale military offensive, although there are differing estimates to how likely a Russian offensive is and what may trigger it. “This is the largest Russian military deployment completely out of cycle that we’ve probably seen period, certainly since 2014,” said Michael Kofman, the research programme director in the Russia studies programme at the CNA security thinktank. Kofman said the size of the build-up, the second this year, unusual troop movements and efforts to prepare reservists was far more elaborate than required to send a credible threat to Ukraine and its western allies. “You could intimidate or scare people with a much smaller force, much more visibly deployed,” he said. The Russian government has accused Washington of creating war hysteria, but Putin has publicly directed government officials to maintain high levels of tension with the west to make sure that its interests are not ignored. Ukrainians have strongly opposed having their fate dictated by the Russian government and officials have called for more military and economic support from the west. Fyodor Lukyanov, a prominent Russian foreign policy analyst, said he did not believe Russia was imminently preparing to launch an offensive. But Moscow had shown that it was ready to use force if it could not negotiate a change to the post-cold war security arrangement in Europe, he said. “There is a real red line,” he said. “Right or wrong. But Russia perceives any kind of military alignment between Ukraine and the west, not necessarily Nato membership … That is seen here as absolutely unacceptable.” He said Putin had shown a personal interest in the conflict’s outcome. “He’s not under time pressure because I don’t see any signs that he is going to leave soon,” said Lukyanov. “But he sees his duty as president not to leave this problem for the next leadership.” There are modest expectations for what Biden and Putin can achieve in Tuesday’s video call, and much will depend on whether Russia at least halts its military buildup following the negotiations. “I’d love them to walk out of this and see Russian forces start to pull back, but I think the Russian forces are going to stay where they are until there’s more of a deal because they are Russia’s leverage,” said Olga Oliker, Crisis Group’s programme director for Europe and central Asia. The considerable Russian forces arrayed on Ukraine’s border were, she said, “reminding you that the unthinkable is there”. The decision of whether or not to engage with the Kremlin in negotiations has elicited strong opinions in Washington and in European capitals, where policy is often presented as either one of strict deterrence or appeasement. Lukyanov said that just an agreement to hold further talks on European security could be viewed as a success in Moscow. But after threats of an invasion for the second time since April, pressure is also growing on the Kremlin to walk away with a concrete win or risk seeing its threats disregarded in the future. “Just to de-escalate for nothing now would be a loss,” said Kofman.