Mushroom Cloud Reported Over North Korea
South Korea's Yonhap new agency reported the incident on Sunday despite the event having taken place on Thursday.
Yonhap, quoting an unnamed diplomatic source in Seoul as saying a mushroom cloud, with a radius of 3.5-4 kms (2.2-2.5 miles), was spotted in Kimhyungjik county in Yanggang province.
Satellite images of the event have gone explained until the Yonhap report.
Seoul's top negotiator with North Korea, Unification Minister Chung Dong-young, said South Korea was seeking confirmation of the explosion, its cause and effects, according to the Yonhap report.
North Korea is a secretive nation that rarely reveals happenings within the country. The press and media generally is tightly controlled by the government.
Earlier this year North Korea admitted to an explosion at a railway station, which killed more than 150 people, three days after the event. Unconfirmed reports at the time put the death toll at much higher levels.
Meantime North Korea is reportedly attempting to delay talks that were planned for later this month on North Korea's nuclear weapons program.
During the last talks in June, South and North Korea, the United States, China, Japan and Russia agreed to meet again before the end of September.
Pyongyang is now seeking to delay the negotiations until after the U.S. presidential race, hoping President George W. Bush is voted out of office, the Korea Times reported Saturday.
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South Korean media has been full of speculation about the blast
BBC report September 13 (note BBC's excellent database, further information, resource links, etc.): South Korean media has been full of speculation about the blast. Visiting UK Foreign Minister Bill Rammell, urged North Korea to allow a British diplomat to inspect the scene. Copy of report here - in full:
North Korea has given its first explanation for the huge blast last week which prompted speculation that it had carried out a nuclear test.
The country's foreign minister, Paek Nam-sun, said the blast was in fact the deliberate demolition of a mountain as part of a huge, hydro-electric project.
His remarks came in response to a call for information by the visiting UK Foreign Office minister, Bill Rammell.
North Korea had said nothing about the incident until now.
After meeting with Mr Paek, Mr Rammell urged North Korea to allow a British diplomat to inspect the scene.
Mr Rammell welcomed the fact that North Korea had provided an explanation.
"But if they are going to be open and engage with the international community, what we really need is diplomats to be able to go to the area and confirm for themselves that that is the case," Mr Rammell said.
The North Koreans have promised to consider the request, he said.
The United States and South Korea had already played down suggestions that the explosion in Yanggang Province, close to the Chinese border, was caused by a nuclear device.
"There was no indication that was a nuclear event of any kind. Exactly what it was, we're not sure," US Secretary of State Colin Powell told ABC television on Sunday.
The blast in Yanggang Province, close to the Chinese border, is said to have happened on Thursday as the Stalinist state celebrated its National Day.
It created what officials in Seoul say was a peculiar-shaped cloud.
The incident and the fears it has provoked around the world are another illustration of the enormous tension between the regime and the international community, says the BBC's diplomatic correspondent James Robbins, in Pyongyang.
North Korea is under international pressure to end all nuclear programmes and disarm.
But so far it has offered only limited concessions during "six-party" international talks involving both Koreas, the US, China, Russia and Japan.
It has not yet committed to attend a fourth round of the talks, which the Chinese, as hosts, wanted to start before the end of this month.