Air samples gathered last week contain radioactive materials that confirm that North Korea conducted an underground nuclear explosion
The same statement confirmed a yield of less than 1 kiloton, a very small explosion.
One of the realities of nuclear weapons is, it is more difficult to develop small yield weapons than the larger ones. The low level yield indicates one of two options, either the test was a failure, or N. Korea is trying to develop low yield nuclear weapons. Since even suitcase nucs are expected to yield up to 2 kilotons, (the US verion of this, the 'backback nuc' was expected to yeild a 1 kiloton blast.), an explosion yield at this level clearly looks like a failure.