The United States and Britain on Sunday flagged new and “devastating” economic sanctions against Russia, as Washington and its NATO allies step up efforts to deter any invasion of Ukraine. Fears of an imminent invasion have grown in recent days, despite denials from Moscow and pleas from Ukraine’s president to avoid stirring “panic” over the massive Russian military build-up on the border. In Washington, the chairman of the Senate Foreign Relations Committee said it was crucial that the United States send a powerful message to Russian President Vladimir Putin that any such aggression would com…