North Korea is reportedly developing a vertical launching system for submarine-launched ballistic missiles (SLBM).
A South Korean Defence Ministry official was quoted by Agence France-Presse as saying: "Based on recent US and South Korean intelligence, we have detected signs of North Korea developing a vertical missile launch tube for submarines."
Speaking at a regular press briefing, South Korean Defence Ministry representative Kim Min-seok said that North Korea may modify its 3,000t Golf-class submarine to be able to fire medium-range ballistic missiles.
Min-seok said: "There is no confirmed information yet that a North Korean submarine capable of launching ballistic missiles is in operation."
Meanwhile, an official document submitted by the Joint Chiefs of Staff (JCS) was cited by Yonhap News Agency as saying: "But the possibility of a North Korean submarine equipped with an SLBM has been detected recently."
In April 2013, the US Defense Intelligence Agency stated that North Korea had miniaturised a nuclear warhead for integration on a ballistic missile.
North Korea is said to have 78 submarines, the most in the world, followed by the US, China and Russia, according to data from Global Firepower (GFP).
However, most of the North's submarines are outdated Soviet-era and Chinese vessels.