Efforts to bring the United States back into the 2015 deal on Iran’s nuclear program are to step up on Tuesday when Iran and the five world powers remaining in the accord meet in Vienna. US officials will be on hand for indirect talks with Iran, which has refused a face-to-face encounter. Iran says the purpose of the event which is held virtually is for the US to end all economic sanctions as a precondition for renewed negotiations. The US delegation is headed by the administration’s special envoy for Iran, Rob Malley. State Department spokesman Ned Price said talks will be structured around working groups that the Europeans will form with the other parties to the accord.