With a key deadline just hours away, U.S. and European officials said nuclear negotiations were imperiled by deep uncertainty over whether Iranian Supreme Leader Ayatollah Ali Khamenei would sign off on the necessary concessions for a deal. All parties to the talks have set March 31 as the date for concluding a framework agreement that would outline all the main elements of a deal constraining Iran’s nuclear program in exchange for lifting international sanctions. The Obama administration and its negotiating partners have said technical discussions on a final, comprehensive agreement will continue through June. But after more than 18 months of direct negotiations, Western officials said there are signs Mr. Khamenei hasn’t empowered his negotiators to give ground on the […]