Secretary of State John Kerry warned Iran on Sunday that hard choices were still needed to seal a landmark nuclear accord, and that the United States was prepared to walk away if a sound agreement could not be reached. “We are not yet where we need to be on several of the most difficult issues,” Mr. Kerry said in a statement in front of the Palais Coburg, the Vienna hotel where the talks are being held. “This negotiation could go either way.” Mr. Kerry’s remarks came two days before a target date for wrapping up the agreement and as foreign ministers from the other world powers involved in the talks are heading here for what is intended to be […]