North Korea conducted a test of its underwater nuclear weapon system in response to the United States, Japan and South Korea's "joint military exercises," state-run media Korean Central News Agency said on Friday.

In a surprise disclosure, North Korean leader Kim Jong-un has been photographed touring a key nuclear arms facility, offering the world its first inside look at the regime’s most secretive program.

North Korea provided a rare glimpse Friday of a secretive uranium-enrichment facility where it produces material for its nuclear weapons program when it released several images of its leader Kim Jong Un touring the site.