Politico is carrying an article that suggests satellite imagery is showing North Korea could be well advanced in its plans for a nuclear test:

The images of the Punggye-ri site, where nuclear tests were conducted in 2006 and 2009, reveal that over the past month, roads have been kept clear of snow and possible steps taken to seal the tunnel into a mountainside where a nuclear device would be detonated.
But it remains difficult to discern North Korea’s true intentions, as a test would be conducted underground.

Of course, this could well be posturing by the North so as to try to enhance their bargaining chips in negotiations.