A newly drafted United States nuclear strategy that has been sent to President Trump for approval would permit the use of nuclear weapons to respond to a wide range of devastating but non-nuclear attacks on American infrastructure, including what current and former government officials described as the most crippling kind of cyberattacks.

Iran’s top nuclear negotiator was heading back to Tehran on Sunday to consult with his nation’s leadership, as negotiators remained divided over how to limit and monitor Tehran’s nuclear program and even on how to interpret the preliminary agreement they reached two months ago.