WASHINGTON—The U.S. military may continue to train Afghan security forces, but do it in other countries after American forces leave Afghanistan, the top U.S. military officer said Thursday, calling it one of several options the Pentagon is considering. Speaking at a Pentagon press conference, Gen. Mark Milley, chairman of the Joint Chiefs of Staff, also said the United States has not settled on a plan yet to continue supporting the Afghan Air Force, which is heavily dependent on the United States for maintenance, training, and repairs. He said some U.S. aid may have to be done from outside the country, but officials are also looking at whether some contracts may be turned over to Afghan control. Defense Secretary Lloyd Austin acknowledged that continuing without American support on the ground “will be a challenge” for the Afghans as they try to hold off Taliban insurgents. This was the first news conference …