Per Fox news, (I think this came from James Rosen) according to a high ranking U.S. official in the Bush administration, there was a test. But the N. Koreans were expecting the results of the explosion to be 400 killitons. They did not get the results they had hoped for. It was of 3.58 magnitude.
The guy on FNC said that 400 kilitons is a ridiculous output-- 20x the yield dropped on Nagasaki. NK, according to SK's knowledge of US intel, has already test nuked Alaska-- managing to get a long-range missile there. Anything close to a Nagasaki yield in Alaska would be a real issue.