Satellite imagery shows increased activity at North Korea's main test site in Punggye-ri, and the defense minister noted that the readiness level is such that "North Korea can conduct an atomic test whenever it makes a decision."
Analysts were unable to determine whether highly-enriched uranium or plutonium was used in the last nuclear test, conducted underground in February 2013. Nuclear tests in 2006 and 2009 likely used plutonium.
Although the South Korean defense minister stated that a nuclear test could be imminent, he acknowledged that "North Korea is very good at psychological manipulations ... Thus, we cannot rule out the possibility that the North could intentionally delay (the nuclear test) or employ a deceptive tactic simultaneously."