By Kim Miyoung and Paul Eckert
SEOUL (Reuters) - A huge explosion rocked North Korea near the border with China three days ago, producing a mushroom cloud that sparked speculation Pyongyang might have tested an atomic weapon, Yonhap news agency reported on Sunday.
The South Korean agency said the blast on Thursday in Kimhyungjik county in Ryanggang province appeared much bigger than a train explosion that killed at least 170 people in April.
South Korean Unification Minister Chung Dong-young sought to play down an atomic link, telling South Korean reporters after a National Security Council meeting that Seoul's assessment so far was the explosion was unlikely to have been part of the communist North's nuclear arms ambitions.
"There are some foreign media reporting such possibilities, but we are judging at the moment the explosion is unrelated to such reports," Yonhap quoted him as saying. Chung chairs the National Security Council, which advises President Roh Moo-hyun.
There was no immediate reaction from neighboring China. In Washington, a U.S. official said it was unclear what had happened and there were various possible explanations. Tokyo took a similar line.
"We've heard the report, and we are checking the details, including what's in the report itself," said Japanese Foreign Ministry spokesman Akira Chiba.
Thursday was the 56th anniversary of North Korea's founding. The reclusive communist state often stages extravaganzas and big events to mark important anniversaries.
South Korean intelligence officials said they were monitoring the news, but declined detailed comment on the reports, which were based on "informed sources" in Beijing and in Seoul. Yonhap did not give a description of the blast site.
ACCIDENT OR TEST?
The reports surfaced as South Korea, Japan, China, Russia and the United States were seeking to persuade North Korea to return to the negotiating table to discuss its nuclear weapons ambitions. The North, which threatened at earlier talks to test an atomic bomb, has said it doubts more negotiations will help.
"There were rumors that the explosion was much bigger than the one at Ryongchon train station and the United States is showing a big interest as the blast was seen from satellites," Yonhap quoted an unnamed source in Beijing as saying The cause had yet to be determined but the source said Washington was not ruling out the possibility that the blast may be linked to a nuclear test.
China was the last country to set off an above-ground nuclear test, in 1980. It carried out its last nuclear test in 1996 and has since observed a self-imposed moratorium on testing.
Yonhap quoted other unnamed officials as saying it was probably not an accident, although it also quoted one source in Washington as saying it was unlikely to have been a nuclear test. It quoted another source as saying it could be a forest fire.
Yonhap reported a mushroom cloud up to 2.5 miles in diameter was spotted after the blast in remote Ryanggang province in the country's far northeast near to known missile bases.
The New York Times reported in its Sunday editions the Bush administration had received recent intelligence reports that some experts believed could indicate North Korea was preparing to conduct its first nuclear weapons test explosion.
Train wagons exploded at the Ryongchon railway station on April 22, killing 170 and injuring an estimated 1,300. The blast was believed to have been caused by a train loaded with oil and chemicals hitting a power line. (Additional reporting by Arshad Mohammed in Washington and Isabel Reynolds in Tokyo)