It's unclear how capable North Korea's submarine weaponry is

Analysis by BBC South Korea correspondent Stephen Evans:

Claims about North Korea's military capabilities always have to be treated sceptically. The latest pictures in the state-run media may or may not be photo-shopped, perhaps to insert Kim Jong-un very prominently into the picture of a missile launch.
However, analysts at the respected US-Korea Institute at Johns Hopkins University in the US reported recently that satellite pictures indicated that "the conning tower of a new North Korean submarine first seen in July 2014 houses 1-2 possible vertical launch tubes for either ballistic or cruise missiles".
The American academics also said that satellite imagery indicated "that North Korea has been upgrading facilities at the Sinpo South Shipyard in preparation for a significant naval construction program, possibly related to submarine development".
Missiles launched from submarines dramatically change the calculation that any potential target must make because the warning time is so much shorter than with land-based missiles with which preparations on the ground might also be detected.
The best estimate of North Korea's nuclear arsenal is that it could make up to 20 weapons but that its ability to make them small enough for a missile is unclear. It is clear, however, that the ambition is there - not least because the North Korean foreign ministry said so: Its nuclear forces were a "powerful, treasured sword" to "protect the sovereignty of the country".
It may be moving faster than previously thought.
-- 9 May 2015