Russia is sending thousands more troops to its border with Ukraine in a sign that Vladimir Putin could extend the crisis for weeks, as Boris Johnson warned the situation had become “very, very dangerous”. British officials estimate that a further 14 Russian battalions are heading towards Ukraine, each numbering about 800 troops, on top of the 100 battalions massed on the borders – a force already believed capable of launching an invasion. Ministers are of the view that the Russian president has not yet decided to attack Ukraine and may never do so. But the continued buildup of forces in excess of 150,000 prompted Johnson to cut short a trip to Cumbria to chair a Cobra emergency meeting on Tuesday. On Monday night, the prime minister spoke with Joe Biden and concluded, according to Downing Street, that there “remained a crucial window for diplomacy and for Russia to step back from its threats towards Ukraine”. Britain said the two emphasised the importance of unity and, while neither the UK nor the US will send troops to defend Ukraine, they insisted that any further Russian incursion “would result in a protracted crisis for Russia”. Earlier, Johnson said: “This is a very, very dangerous, difficult situation. We are on the edge of a precipice, but there is still time for President Putin to step back.” He said it was important for western allies to remain united in the face of Russian pressure. Ukraine’s future membership of Nato could not be “bargained away” as part of western leaders’ efforts, he added. Britain believes Russia has committed about 60% of its ground forces to the buildup and doubled its available air power in the region, but thinks Putin could extend the crisis for weeks or even longer. Although Liz Truss, the foreign secretary, has said “there could be a Russian invasion almost immediately”, on Monday she told MPs in a private conference call that Putin could keep up the military pressure on Ukraine for months, according to some of those who listened in. Parliament could be recalled if Russia invades Ukraine this week, No 10 indicated. Asked whether this was a possibility, Johnson’s official spokesperson said: “In that situation, you would expect that the prime minister would want parliament to be updated and for it to have its say.” Russia has always denied that it intends to invade. Its foreign minister, Sergei Lavrov, told Putin in a carefully staged public meeting on Monday that while negotiations between Russia and the west could not “go on indefinitely … I would suggest to continue and expand them at this stage”. Olaf Scholz, Germany’s chancellor, is due to visit Moscow on Tuesday, although no breakthrough is expected given Russia’s demands: that Ukraine never be allowed to join Nato and that the west withdraw troops from eastern Europe. Last week the US told allies that an invasion could come as soon as Wednesday, but the continued Russian buildup suggests the Kremlin did not believe it had assembled all the forces it could to threaten its smaller neighbour. Ukraine’s president, Volodymyr Zelenskiy, said his country would not be intimidated by Russia and that Wednesday should be “a day of unity”. “We want peace and we want to resolve all issues exclusively through negotiations,” he said, but added that the Ukrainian army was “many times stronger than eight years ago” and that Ukraine wanted freedom and was ready to fight for it should Russia invade. Nevertheless, the warnings prompted the UK to advise all British citizens in Ukraine to leave now, while commercial means were still available. It is estimated there are about 1,300 Britons in Ukraine, although 200 have said they do not want to leave. On Monday, the US secretary of state, Antony Blinken, said the last few US diplomats in the Kyiv embassy would move west to Lviv “due to the dramatic acceleration in the buildup of Russian forces”. On Saturday, a senior state department official had told reporters that a handful of US diplomats would stay in the capital “to be able to continue working closely with the Ukrainian government, and to be able to ensure we’ve got the best possible information for our senior leaders and the president about what’s happening broadly in society”. The change of mind two days later suggests US security assessments have darkened still further. The UK ambassador to Ukraine, Melinda Simmons, had said she would remain in Kyiv with a core team of diplomats. Downing Street said British nationals should not expect a “military airlift” from Ukraine, and that they should leave via commercial means and there were still flights available. Defence sources said the plan for any Russian invasion would involve “a sudden thrust” aimed at encircling Kyiv, but that it would be accompanied by “multiple axes of attack” aimed at cutting off Ukrainian forces in the east of the country. Russia would seek to install a puppet government, they added. But Britain believes Putin is at risk of underestimating the level of Ukrainian popular resistance to any invasion and attempt at regime change. The UK view is that there is likely to be “a reasonably well-organised insurgency” and that Russia could become bogged down in a conflict that could claim tens of thousands of lives. The US, UK and EU have been working jointly on a package of economic sanctions that would target Russia in the event of an attack, and Truss indicated to MPs in the briefing that they would probably be implemented in waves. Oligarchs close to Putin would be the first to be targeted, she said, telling MPs the UK was “ready to press send” with list of names if an invasion began. But an economic crime bill planned for the next session of parliament, called for by Labour and the Liberal Democrats, would not be brought forward in an emergency. Layla Moran, the Lib Dems’ foreign affairs spokesperson, said: “The concern with the policy on sanctions is that it is too little, too late. We have known about Russian hostile activities and interference in our democracy for some time.”