Seoul, Jan 19 : North Korea on Friday said it has tested an underwater nuclear weapon system under development in response to the latest joint maritime exercise between South Korea, the US and Japan. According to Pyongyang's official Korean Central News Agency (KCNA), the North Korean Defence Ministry said it conducted an important test of the "Haeil-5-23" in the East Sea, denouncing the drills as "reckless" military confrontation hysteria, reports Yonhap News Agency. "Our army's underwater nuke-based countering posture is being further rounded off, and its various maritime and underwater responsive actions will continue to deter the hostile military maneuvers of the navies of the U.S. and its allies," a spokesman of the Ministry said in a statement carried by the KCNA. South Korea, the US and Japan jointly conducted naval drills involving the US nuclear-powered aircraft carrier USS Carl Vinson between Monday and Wednesday, following Pyongyang's latest launch of a hypersonic missile. /IANS