(LEAD) U.S Air Force chief: N.K. yet to master technology to put nuclear warhead on long-range missile

(ATTN: UPDATES with Pentagon briefing in last 5 paras) WASHINGTON, March 8 (Yonhap) -- North Korea is not believed to have mastered the technology to fit a nuclear warhead onto a long-range missile yet, U.S. Air Force Chief of Staff Gen. Mark Welsh sa