South Korea, Japan and the United States staged long-planned joint naval exercises involving an American aircraft carrier to ensure readiness against nuclear and missile threats from North Korea, South Korea's navy said on April 12.The two-day drills held in international waters between South Korea and Japan from April 11 brought together the air carrier Theodore Roosevelt and the destroyers Howard, Russell and Daniel Inouye, with warships from the Asian neighbours. The U.S. military released photographs of the trilateral drills.The training, aimed to step up their combined capability to respond to North Korea's threats, follows a multi-year joint exercise plan set up after last year's three-way summit, the South Korean Navy said.North Korea has been accelerating weapons development, testing solid-fuel hypersonic missiles last week, and leader Kim Jong Un said on April 10 now was the time to be more prepared for war than ever, citing an unstable geopolitical situation.