Apparently, they detonated a ~10KT nuke underground (at least based on the 5.1 magnitude earthquake that centered around the test site).
Personally i'm always interested in what the Russian, Chinese, and Iranian responses are to things like this since the west might as well recycle the same responses over and over.
Russian response:
Chinese response:In a statement, Russia's Foreign Ministry demanded that North Korea refrain from any nuclear missile program and adhere to U.N. Security Council guidelines.
It condemned the test as an affront to the community of nations: "It's doubly sad that we are talking about the state with which our country has a long history of good neighborliness."
The Chinese Foreign Ministry said it "resolutely opposes" the North's latest test, which it noted had taken place "despite the international community's widespread opposition."
Beijing summoned the North Korean ambassador to China over its "dissatisfaction" with the test, the ministry said.
It said it "strongly" urged North Korean officials to "abide by their promise to denuclearize and take no further action that will worsen the situation."
http://www.cnn.com/2013/02/11/world/...html?hpt=hp_t2
Iran's response:
"We think we need to come to a point where no country will have any nuclear weapons," Iranian Foreign Ministry Spokesman Ramin Mehmanparast told a weekly news conference when asked about the test. "All weapons of mass destruction and nuclear arms need to be destroyed."
http://www.reuters.com/article/2013/...91B0KC20130212