Russians and Chinese are much more concerned about this then the US is and rightfully so because if the sh*t hits the fan, its going all over those two.
http://www.bbc.co.uk/news/world-asia-pacific-12029875
Due to existing alliances, the US will automatically goto war with the NK if SK is attacked. We also do have a 57 year old UN alliance with SK, but whether we would send troops seems highly unlikely given our current military status.
http://www.cbc.ca/politics/story/201...-military.html