SEOUL, Oct 19 (Reuters) - North Korea test fired what appeared to be a submarine-launched ballistic missile (SLBM) on Tuesday, South Korea said. It would be the latest step in nuclear-armed North Korea"s efforts to field SLBMs, which are seen as a potent nuclear deterrent because they can be difficult to detect or destroy. Analysts and officials in South Korea believe the country is building an operational ballistic missile submarine. Here are the SLBMs that North Korea has displayed or tested, usually from submersible barges rather than from a submarine. PUKGUKSONG-1 Starting in 2014, North Korea has conducted five flight tests of the KN-11, and well as numerous other tests of the missile"s ejection mechanism and other subsystems, according to the U.S.-based Center for Strategic and International Studies. Several of the first tests appeared to fail, before a first successful test in 2016. Analysts estimated the missile"s range to be about 1,200 kilometres. North Korea has also test fired a Pukguksong-2, considered a land-based variant of the Pukguksong-1. PUKGUKSONG-3 Before Tuesday, North Korea"s last SLBM test was in October 2019, when a Pukguksong-3 missile was fired from an underwater platform, flying 450km to a maximum altitude of 910km. Like its predecessor, the Pukguksong-3 appears to have two stages powered by solid fuel. The missile has "a cold-launch" system that uses pressurized gas to eject it from a submarine or other launch tube. "After surfacing, the missile ejects its rear engine cover and ignites its main booster," CSIS says in a summary of the Pukguksong-3. PUKGUKSONG-4 At a military parade in October 2020, North Korea unveiled a previously unseen SLBM that was dubbed the Pukguksong-4. It appeared to be about the same size as the Pukguksong-3, and its capabilities remain unclear. "If the new SLBM is intended to be deployed, it may be intended for the new conventionally powered ballistic missile submarine that North Korea hinted at building in July 2019," analysts at the U.S.-based 38 North programme, which tracks North Korea, said at the time. PUKGUKSONG-5 Another previously unseen SLBM, designated the Pukguksong-5, was displayed at a military parade in January. Few details on its capabilities could be gleaned from its outward appearance, and its dimensions appeared to be comparable to earlier designs, 38 North said in a report. "These dimensional similarities indicate North Korea is still in the process of settling on a specific design for its next-generation SLBM," it said. MYSTERY MISSILE At an unusual military show opened in Pyongyang last week, North Korea displayed a previously unseen missile next to its other SLBMs. The mystery missile is smaller than its other SLBMs, potentially presenting an easier pathway to an operational ballistic missile submarine, analysts said. South Korea has recently demonstrated a conventional SLBM launched from a submarine.