North Korea has unveiled for the first time what it says is a nuclear-powered submarine under construction, a weapons system that can pose a major security threat to South Korea and the US.

North Korea on March 8 released images of leader Kim Jong-un inspecting the construction of what it calls a “nuclear-powered strategic guided missile submarine,” marking its first public acknowledgment of the project.

North Korea has long vied for nuclear-powered submarines, which would allow it to fire missiles from underwater. Advanced detection of the weapons is difficult, making the country’s construction of the warships worrisome.