National security adviser Wi Sung-lac says North Korea appears to be building a submarine that is not only powered by nuclear fuel but is also capable of carrying and launching nuclear weapons.

Wi made the assessment on Friday when asked to comment on Pyongyang releasing images of the nuclear-powered submarine.

He said there is a need to find out more about the capabilities of the submarine, stressing that its existence alone poses a new threat to South Korea. 

The adviser then stressed the need to be prepared and as part of that effort explain to China why South Korea needs to introduce nuclear-powered submarines. 

Wi stressed that the submarines South Korea is aiming to introduce are not in violation of the Nuclear Non-Proliferation Treaty and are not a problem for the International Atomic Energy Agency.