They have now said they are canceling every agreement and accord they ever made with the South and China has sent an "emergency" envoy to Pyongyang.
China does not approve of what NK is doing.
http://www1.voanews.com/english/news/asia/North-Korea-Reneges-On-Naval-Agreement-with-South-95004154.htmlexcerpt
In a sharp escalation of tensions with South Korea, North Korea says it is discarding military procedures the two sides have agreed to follow for years to ensure a conflict does not get out of hand.
The general staff of the North Korean People's Army issued a notice Thursday dismantling a wide range of security guarantees it has observed for years.
Pyongyang says it will completely nullify a bilateral agreement with the South that was put in place to prevent clashes in waters west of the Korean peninsula. That is the maritime region where North Korea has disputed a sea border for decades and where an international team of investigators concluded a North Korean submarine sank a South Korean patrol ship in March, killing 46 sailors.
The North says it will stop using maritime shortwave radio frequencies with the South and cut off other means of emergency communication.
That warning came as South Korea's navy conducted anti-submarine warfare drills off its west coast. South Korea and the United States have announced they will conduct a joint anti-submarine exercise, widely expected to take place within a matter of weeks.
The North Korean military notice, carried on state-run media, also said South Korean ships and aircraft will no longer be allowed passage through the North's territorial space. Pyongyang says it may also prevent access into or out of a joint industrial complex in the North Korean city, Kaesong, potentially trapping hundreds of South Korean business managers there
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