SEOUL, April 16 (Yonhap) -- South Korea and the United States will kick off a large-scale regular joint air exercise later this week, the South's Air Force said Sunday, in the latest drill meant to step up deterrence against North Korea's evolving military threats. (PHOTO NOT FOR SALE) (Yonhap)The South's Air Force said the drills will take place with a focus on enhancing the interoperability and combined operational capability of the allies' advanced fourth and fifth generation fighters, such as the F-15K and radar-evading F-35 aircraft, respectively. The allies have staged a series of combined air exercises this year amid Pyongyang's continued saber-rattling