N Korea has always run her mouth. The reason the US is taking it more "seriously" now is an advancement of goals in the Pacific theater...especially regarding China. Tensions in the last couple of years have risen to the point that mainstream media is calling it a new "cold war"...and China has been exponentially increasing military strength and capability the last few years combined with dumping dollars and buying rare earth materials at a phenomenal rate.
All-out war is unlikely; however, skirmishes and regional warfare that indirectly involves China (N Korea) is VERY likely...almost unavoidable at this point.
Powder Keg in the Pacific