Analysts in Seoul say the North Korean leader Kim Jong Il is trying to mark his country's presence as a full nuclear power.
"North Korea is making a very clear statement that we are a nuclear state. Now deal with us, as such," said Lee Jung-Hoon, professor of international relations at Yonsei University in Seoul. "It's a wake-up call to the international community that we shouldn't pretend as if somehow North Korea would negotiate dismantlement."
Although it was expected, the timing came as a surprise. U.S. officials did say last week that they were monitoring activity at the nuclear test site but said it was difficult to determine whether a test was imminent given that a lot of that work was underground. Officials also detected activity at the nearby Musudan-Ri, where the long-range Daepodong 2 missile was launched April 5. No missile has been seen there, but the work that's been under way indicates another launch is in the works, according to U.S. officials.
"You can tell that Kim Jong-Il seriously feels that there's not much time left. He'll use whatever means he could to increase the bet," said Choi Jinwook, senior researcher at Korea Institute for National Unification.
"North Korea is a bully. If you concede, it will continue to push, and it's pushing and pushing to the point where now, we can't almost go back," said Lee implying that a harder stance on North Korea is needed to get it to give up the nuclear program. "It's going to be a huge headache, not only for the U.S. but also for the international community as a whole."
ABC News' Luis Martinez and Noriko Namiki contributed to this story.