Apparently North Korea has declared that any sactions enacted against them "cannot be construed as anything but a declaration of war," and that they won't sit idly by now that they are a nuclear power.
Of course, this is part of why the DPRK wanted a nuclear weapon, to be able to claim nuclear power for leverage and negotiation purposes.
Now, to be fair, North Korea construes just about anything, from sanctions to dipping your donut in your coffee, to be construed as a declaration of war. Anyone who's read the Pyongyang times at any point over the past 15 years would likely have seen the press in North Korea referring to various acts, such as Tony Shalhoub winning an emmy for Monk, as declarations of war. (note, I am a big fan of Tony Shalhoub, and absolutely love Monk - one of the few well written shows left on television).
North Korea is a formiddable force. When I inprocessed at Osan Air Base, the inbriefers all started their briefs with "welcome to Osan Air base, four minutes from death." The DPRK has enough firepower collected at the DMZ to level Seoul rather quickly (the South has similar force built up on their side - it's the most heavily armed area in the world), so they are a group to be dealt with. But it's hard to take rhetoric like this statement seriously when it's been used SO MUCH over the years.