Monday's explosion dashed hopes that the secretive nation is simply building its weapons program as a bargaining chip.
Monday's explosion – the latest in a series of hard-line moves by Pyongyang – may have been 20 times more powerful than its last test in 2006.
Senior official Choe Sung-chol was reportedly put to death for accepting bribes.
US envoy Stephen Bosworth is visiting Beijing, Seoul, and Tokyo, after Pyongyang rejected six-party dialogue and threatened to test another nuclear bomb.
United by a common enemy, South Korea's Navy moves to protect a North Korean cargo ship.
Pyongyang may fulfill a vow to conduct another nuclear test, analysts say. It may also be testing US response.
North Korea focused on labor rates in joint industrial complex, while South Korea sought freedom for jailed worker.