DALLAS—Several leading U.S. airlines warned Thursday that the rise in COVID-19 cases due to the delta variant is hurting their bookings and further delaying the travel industry’s recovery. The summer got off to a strong start, with many planes full of vacationers eager to break out after being stuck at home for more than a year. After months of improving travel numbers, however, August was disappointing. Delta Air Lines CEO Ed Bastian said Thursday that people are still traveling, but key segments— business and international flyers—are still largely missing. He said the rise in COVID-19 cases won’t derail the travel recovery but will delay it by 90 to 120 days. Delta said it still expects to post an adjusted pretax profit for the third quarter, but revenue will be toward the lower end of its previous forecast. United Airlines said its revenue is weaker than previously expected, and it forecast …