The North Korean military conducted a test of its "Haeil" underwater nuclear drone system in response to joint military drills by the U.S., Japan and South Korea, according to DPRK state media on Friday.

"The Underwater Weapon System Institute under the DPRK Academy of Defense Science conducted an important test of its underwater nuclear weapon system 'Haeil-5-23' under development in the East Sea of Korea," the Korean Central News Agency (KCNA) reported, without specifying when the test took place.