Joe Biden on Wednesday night condemned as a “big mistake” Russian president Vladimir Putin’s decision to suspend his country’s participation in the last remaining US-Russia nuclear arms control treaty. The comment came as the US president closed out his wartime visit to Europe, working to shore up partnerships with allies on Nato’s perilous eastern flank – even as Putin was drawing closer to China for help as his invasion of Ukraine neared the year mark. Biden’s meeting with leaders of the Bucharest Nine nations in Warsaw came at the conclusion of a whirlwind, four-day visit to Ukraine and Poland meant to reassure allies that US support in fending off Russia isn’t at risk of waning. In dramatic counterpoint, Putin on Wednesday played host in Moscow to Wang Yi, the Chinese Communist party’s most senior foreign policy official, as US intelligence warned that Beijing is considering supplying arms and ammunition to the worn-down Russian military. The flexing of alliances was a fresh indication that both sides are digging in for prolonged conflict in Ukraine with the fighting expected to intensify with the arrival of spring. Biden had delivered a ringing speech on western unity in Warsaw, a day after he swept into Kyiv unannounced for a visit with Ukrainian president Volodymyr Zelenskiy. As Biden spoke in Poland, Putin announced that Russia was suspending its participation in the last remaining US-Russia nuclear arms control treaty. Biden has now called that departure “a big mistake”. The exit is expected to have an immediate impact on US awareness of Russian nuclear activities. However, the pact – known as New Start – was already on life support following Moscow’s cancellation late last year of talks that had been intended to salvage an agreement that each side has accused the other of violating. “You’re the frontlines of our collective defense,” Biden told the Bucharest Nine, which comprises Bulgaria, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia. “And you know, better than anyone, what’s at stake in this conflict. Not just for Ukraine, but for the freedom of democracies throughout Europe and around the world,” he added. These countries have worried that Putin could move to take military action against them next if he’s successful in Ukraine. Biden responded to that anxiety by pledging that Nato’s mutual defense pact is “sacred” and that “we will defend literally every inch of Nato”. Nato secretary general Jens Stoltenberg, who attended the meeting, pointed to past Russian conduct in Georgia and Ukraine and said: “We cannot allow Russia to continue to chip away at European security. We must break the cycle of Russian aggression.”