But given the recent concern over North Korea's nuclear ambitions, news of the explosion -- near a missile base -- sparked widespread anxiety.
Today, North Korea tried to play down that concern. Foreign Minister Paek Nam-sun told a visiting British diplomat, Bill Rammell, that the blast was a controlled explosion conducted as part of a hydroelectric power project.
In Seoul, a senior research associate at the Center for Nonproliferation Studies, Daniel Pinkston, said it was too early to draw any definitive conclusions, but he said it could have been an accident. "If it were a nuclear test, a nuclear explosion, I think we would know by now," he said. "There would be some announcement, because there are devices to detect the fallout and so forth from a test."
South Korea's Yonhap news agency first reported the explosion and accompanying mushroom cloud on 9 September. It took place in northern Yanggang province, near the Yongjori missile base. The agency said a crater at the blast site was large enough to be seen by satellite surveillance. The blast was apparently stronger than an April explosion that killed 160 people and injured an estimated 1,300 at a North Korean railway station when a train carrying oil and chemicals apparently hit power lines.
The explosion came on the anniversary of the founding of North Korea in 1948 -- a date that the North characteristically marks with a show of military force.
South Korean newspapers today were filled with speculation about what might have cause the blast. The "Joong Ang Ilbo" newspaper quoted a South Korean source as saying the blast was probably caused by an accident at a military factory or an underground munitions depot. Another South Korean daily, "Chosun Ilbo," cited a North Korean defector familiar with the area as saying chemical materials might have exploded.
The South Korean government yesterday played down a nuclear link. South Korean Unification Minister Chung Dong-young told reporters Seoul's assessment so far was the explosion was unlikely to have been part of North Korea's nuclear arms ambitions.
U.S Secretary of State Colin Powell also rejected speculation yesterday that North Korea was carrying out nuclear tests. "They haven't conducted [nuclear] tests to the best of our knowledge and belief," he said. "And the activity reported [this week] is not conclusive [about] if they are getting ready to do one."
The United States, Russia, Japan, China, and the two Koreas have held talks on North Korea's suspected nuclear-weapons development, and they agreed to hold another round of negotiations in Beijing this month. No date has been set.
The United States has pushed for North Korea to fully disclose all of its nuclear activities and allow outside monitoring before it receives any assistance. North Korea wants energy aid, lifting of economic sanctions, and removal of its inclusion on Washington's list of state sponsors of terrorism.