From what the media knows, the United States allegedly broke the boundaries that North Korea has set. Supposedly "thrice" is the amount of times the US has supposedly broke their rules, says the head of the ministry. From what I know, the North Koreans are at a state where tension is breaking their relations with foreigners on their boundary and recent visitors. The country, which was formerly disconnected from the outer-world entirely is supposedly saying that the American's flaws indirectly represent the declaration of war and not an official declaration. These forms of petty arguments have been going on ever since the US's attempts in denuclearizing the state entirely. Hopefully this does not mean a start to a war with Korea and that matters are settled with the upcoming month, which was and might still be some sort of "war games" as North Korea describes. But rest assured that no official declaration has been made, and that this seems to be solely to add tension to the United States.
Please read here for more details:
http://www.usatoday.com/story/news/world/2016/07/28/north-korea-united-states-relations/87659264/