This could be a nuclear test, or it could be another accident similar to the one which levelled Ryongchon in April. We won't know until the CTBTO's international monitoring network picks up fallout.
Update: According to CNN, an unnamed US official has denied it was a nuclear test. His excuse though is a classic:
The U.S. official said the cloud could be the result of a forest fire.
I'm sure that's Tui-able.