A large explosion occurred in the northern part of North Korea (search), sending a plume of smoke more than two miles wide into the air on an important anniversary of the communist regime, a South Korean news agency reported Sunday.
The explosion happened at 11 a.m. Thursday in Yanggang (search) province near the border with China, according to the Yonhap news agency.
Yonhap carried reports from unidentified sources, with one in Washington saying the incident could be related to a natural disaster such as a forest fire. It also cited a diplomatic source in Seoul as raising the possibility of an accident or a nuclear test.
Yonhap later quoted Kim Jong-min, spokesman for the presidential office, as saying:
"Currently, we are trying to find out in detail the exact character, cause and size of the accident, but we don't think North Korea conducted a nuclear test."
North Korea is believed to be developing nuclear weapons. International experts would likely have been able to detect a test if one had occurred several days ago.
"We understand that a mushroom-shaped cloud about 3.5 to 4 kilometers (2.1 to 2.5 miles) in diameter was monitored during the explosion," the source in Seoul told Yonhap.
The agency also quoted an unidentified government official as saying there was seismic activity related to two blasts in North Korea at 11 p.m. Wednesday and 1 a.m. Thursday.
Thursday was the anniversary of North Korea's founding on Sept. 9, 1948. Leader Kim Jong Il (search) uses the occasion to stage performances and other events to bolster loyalty among the impoverished North Korean population.
Experts have speculated that North Korea might use a major anniversary to conduct a nuclear-related test, but one analyst said an open test, as opposed to one below ground, would be difficult in such a small country.
"It's difficult to say, but it won't be easy for North Korea to conduct a nuclear test without resulting in massive losses of its own people," said Koh Yu-hwan, a North Korea expert in Seoul. "I think there is a (greater) possibility that it is a simple accident, rather than a deliberate nuclear test."
Yonhap's diplomatic source in Seoul said the explosion took place "not far" from a military base that holds ballistic missiles. North Korea, which has a large missile arsenal and more than a million soldiers, is dotted with military installations.
The damage and crater left by the explosion in Kim Hyong Jik county was big enough to be noticed by a satellite, a source in Beijing told Yonhap.
Kim Jong Il on Sunday met Li Changchun, a senior official of China's Communist Party who was on a goodwill visit to Pyongyang, said KCNA, the North's official news agency. Li delivered a letter from Chinese President Hu Jintao, KCNA said.
KCNA did not mention the reported explosion. China had said that the agenda for Li's talks would include North Korea's nuclear development.
On Saturday, North Korea said recent revelations that South Korea conducted secret nuclear experiments involving uranium and plutonium made the communist state more determined to pursue its own nuclear programs.
The South Korean experiments, conducted in 1982 and 2000, were likely to further complicate the already stalled six-nation talks aimed at dismantling the North's nuclear development. South Korea has said the experiments were purely for research and did not reflect a desire to develop weapons.
On April 22, train wagons at a railway station exploded in the North Korean town of Ryongchon, killing 160 people and injuring an estimated 1,300, according to some estimates. The blast was believed to have been sparked by a train laden with oil and chemicals that hit power lines.
The explosion on Thursday was bigger than the Ryongchon train explosion, which devastated a wide area, Yonhap said.