U.S. and South Korean officials say they are still trying to discover the cause of an explosion in North Korea last week - but they say the blast was not nuclear.
Thursday's explosion, which resulted in a huge mushroom cloud, was first reported Saturday by South Korea's Yonhap news agency. It took place in North Korea's Yanggang province, near the border with China.
Yonhap said it appeared stronger than the April rail blast in North Korea that killed 150 people.
Secretary of State Colin Powell said (on U.S. television) Sunday there is no indication the event was nuclear. But he said the United States is closely monitoring reports of increased activity at a potential nuclear-test site in North Korea.
South Korean officials say they have not detected any signs of a nuclear test in North Korea.
Information for this report is provided by AP and AFP.