The Biden administration is very “concerned” that Russia might escalate its invasion of Ukraine and use nuclear weapons, U.S. national security adviser Jake Sullivan said Sunday. In an interview with CNN’s Dana Bash on “State of the Union,” Sullivan said the United States was monitoring Russia’s moves closely but noted that officials have not yet seen anything that would require them to change their “nuclear posture.” “We are watching this extremely closely, and obviously, the escalation risk with a nuclear power is severe, and it is a different kind of conflict than other conflicts the American people have seen over the years,” Sullivan said. “And the American President, Joe Biden, has to take that responsibility extremely seriously, even as we redouble our efforts to support the Ukrainians. As things stand today, the United States has not adjusted our nuclear posture, but it is something that we monitor day by day, hour by hour, …