(2nd LD) Mattis: Reduction in U.S. troops in S. Korea exercises was not caused by N.K. tensions

(ATTN: UPDATES with another North Korean warning, background in last 5 paras)By Lee Haye-ah

WASHINGTON, Aug. 20 (Yonhap) -- U.S. Defense Secretary Jim Mattis said Sunday that this week's joint exercises with South Korea will involve fewer American t