North accuses South of sea invasion

This article, published today, talks about how North Korea is accusing South Korea of sending warships into North Korean waters... and replied with a promise to send the ships to the bottom of the ocean if this continues. South Korea however, is denying any part in this. Apparently these ships started appearing in May, and now there are 36. Makes me wonder why North Korea didn't say something when they saw the first one if this is really true. It seems any country uncomfortable with foreign naval presence in their waters would take the neccessary step without allowing over a month to pass. The waters are noted to be a great area for fishing, and that alone is enough to undestand why both countries would want a stronghold there- one that has caused the deaths of sailors due to the conflict. If the Koreas are trying to move to unification, I think the sea off of the Korean penninsula could be a profitable place for another special economic zone. This would take immense cooperation, and special agreements not to use the waters as a means of a military advance, but shared money making waters would be much better than unclear accusations about invasions.