evidence: President Joe Biden has warned Russia that the United States will not be intimidated by reckless threats after Vladimir Putin declared the annexation of four occupied regions of Ukraine. [...] Nato chief Jens Stoltenberg said the Russian move was "the most serious escalation since the start of the war". [...] Mr Putin last week said his country had "various weapons of destruction" and would "use all the means available to us", adding: "I'm not bluffing."