Joe Biden has said he believes Russia is on the brink of invading Ukraine, as he joined Nato allies in warning that shelling in the disputed east of the country may be an attempt to set up the pretext for an incursion. Claims of attacks by Russian-backed separatists at several locations in Ukraine’s Donbas region, including at a kindergarten and a school, were said to bear the hallmarks of an attempt to incite conflict. The US president, speaking shortly after the expulsion of his country’s deputy ambassador to Moscow, said his administration had “reason to believe” that Russia was “engaged in a false-flag operation to have an excuse to go in”. He told reporters: “Every indication we have is they’re prepared to go into Ukraine, attack Ukraine … My sense is it will happen in the next several days.” The president made his comments as Russia handed over its long-awaited response to American and Nato proposals about European security. The Kremlin said in its 10-page letter that the US had not taken its concerns seriously about Ukraine’s potential to join Nato and that Russia would need to take unspecified “measures of a military-technical nature”. Biden ordered his secretary of state, Antony Blinken, to change his travel plans at the last minute in order to speak at a United Nations security council meeting on Ukraine. Russia continues to deny that it has any intention of invading Ukraine but Blinken told those assembled that they found themselves in a “moment of peril for the lives and safety of millions of people”. He said: “Our information indicates clearly that [Russian] forces, including ground troops, aircraft, ships, are preparing to launch an attack against Ukraine in the coming days. “We don’t know precisely how things will play out, but here’s what the world can expect to see unfold. In fact, it’s unfolding right now. “First, Russia plans to manufacture a pretext for its attack. This could be a violent event that Russia will blame on Ukraine, or an outrageous accusation that Russia will level against the Ukrainian government. We don’t know exactly the form it will take. It could be a fabricated so-called terrorist bombing inside Russia. The invented discovery of the mass grave, a staged drone strike against civilians, or a fake – even a real – attack using chemical weapons. “Russia may describe this event as ethnic cleansing, or a genocide, making a mockery of a concept that we in this chamber do not take lightly.” Before the security council meeting, the Russian mission circulated allegations of war crimes and “genocide” against the people of the Moscow-backed Luhansk and Donetsk separatist republics. Most of the allegations concerned accounts of civilian casualties from Ukrainian shelling, largely in 2014, when the conflict was most intense. The Russian documents referred to “mass graves”, but it was unclear whether they were alleging the burial sites were dug by Ukrainian forces or by the families and communities of the dead during the fighting. The Russian deputy foreign minister, Sergey Vershinin, urged council members to look at the allegations, saying they would be “horrified by them”. He repeated Moscow’s denials of any intention to invade Ukraine, and blamed Ukraine for ceasefire violations The British prime minister, Boris Johnson, echoed the US president’s sentiments, saying the sudden escalation of violence in Ukraine’s eastern territories may be “designed to discredit the Ukrainians”. His foreign secretary, Liz Truss, said she was “very concerned” by Thursday morning’s developments. “Reports of alleged abnormal military activity by Ukraine in Donbas are a blatant attempt by the Russian government to fabricate pretexts for invasion,” she said. “This is straight out of the Kremlin playbook.” Speaking at a meeting of Nato defence ministers, the military alliance’s secretary general, Jens Stoltenberg, and the US defence secretary, Lloyd Austin, said they had long feared that Russia would look to engineer an excuse for a military incursion. Stoltenberg said: “We are concerned that Russia is trying to stage a pretext for an armed attack against Ukraine. It is still no clarity, no certainty about the Russian tensions. “But what we do know is that Russia has amassed the biggest force we have seen … for decades in and around Ukraine. And we also know that there are many Russian intelligence officers operating in Ukraine. “They are present in Donbas, and we have seen attempts to stage pretext, false-flag operations to provide an excuse for invading Ukraine.” Biden has said that about 150,000 Russian troops are on Ukraine’s border. Officials said the crisis was “entering into a very dangerous” period, and that if the shelling in the Donbas region was sustained, and played up on Russian media, it will have entered a new phase. Speaking at Nato headquarters in Brussels, Austin, a retired four-star general, said all the evidence suggested Russia was preparing for an assault. “We see them add to the more than 150,000 troops that they already have arrayed on that border. Even in the last couple of days,” he said. “We see some of those troops inch closer to that border. We see them fly in more combat and support aircraft. We see them sharpen their readiness in the Black Sea. “We even see them stocking up their blood supplies. You know, I was a soldier myself not that long ago. And I know first-hand that you don’t do these sorts of things for no reason. And you certainly don’t do them if you’re getting ready to pack up and go home.” After the emergence of photographs showing buildings damaged by shells, separatists in the Luhansk region claimed they had been forced to return fire in response to Ukrainian shelling, describing it as a “large-scale provocation”. Dmitry Peskov, the Kremlin spokesperson, said the shelling “was a subject of very, very deep concern”, describing the incidents as “provocations”. Kyiv disputed the separatists’ claims, saying that Russian-backed groups had initiated the shelling and that its forces had not fired back. A nursery building in Stanytsia Luhanska was hit, wounding two civilians, according to the Ukrainian military. More shelling of the town was reported on Thursday night, but with no immediate reports of casualties. Ukraine’s president, Volodymyr Zelenskiy, described the actions of the pro-Russian separatists as a “big provocation”. Western security officials closely monitoring the shelling in Donbas said it was “the sort of provocation that has the potential to escalate”, while also stressing that exchanges of fire across the line of control in eastern Ukraine were not uncommon. An official said there was “more concern with what is happening now” than with any possible pretext for an invasion by Russia that had been set out previously.