The New York Times is reporting that the US Govt is increasingly concerned by the apparently increased possibility of a North Korean nuclear test. However, they are also very keen to play down the gigantic explosion reported in NK on Thursday. Of course, there is no possible way a bang that created a mushroom cloud of 2.5 miles' diameter and a crater visible from space could be a nuclear weapon! What else it might be seems unclear - one commenter suggested a large steam locomotive boiler, but I have a little difficulty accepting that in the light of reports of a 3.7 Richter earthquake being associated with it. (German speaking link, referring to Chinese sources) That would be quite a big train, no?
(edit)It is reported that it took place near a missile base - a slightly less scary explanation might be an accident with liquid fuel, as occurred on a couple of occasions in the cold war.