SEOUL, South Korea — The U.S. defense secretary said Wednesday there is evidence that North Korea has sent troops to Russia, calling it a “very, very serious issue” if they join the war in Ukraine on Moscow’s side and warning of possible consequences. South Korea’s spy chief, meanwhile, told lawmakers that 3,000 North Korean troops are now in Russia receiving training on drones and other equipment before being deployed to battlefields in Ukraine. [time-brightcove not-tgx=”true”] The U.S.