A huge explosion rocked North Korea three days ago but US and South Korean officials said this morning it was unlikely to have been a nuclear weapons test despite a report the blast produced a mushroom cloud.
South Korea's Yonhap news agency said the blast on Thursday in Kimhyungjik county in Ryanggang province in the northeast near the border with China appeared much stronger than a train explosion that killed at least 170 people in April.
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.
But in Washington, US officials said there was no definitive explanation yet, although Thursday's blast did not appear to be nuclear.
A test would alter the stakes in the North's standoff with the United States over Pyongyang's atomic ambitions.
"We've got no indication that anything of the sort has happened. We believe these reports to be completely unfounded," said a State Department official who asked not to be named. "People are pretty sure it's not a mushroom cloud and not a test of any kind," the official said.
Yonhap said a mushroom cloud up to 4 km (2.5 miles) in diameter was seen after the blast in an area near missile bases.
"There were indications last week of an incident in the area and we are still trying to confirm its exact nature," said South Korean Defence Ministry spokesman Nam Dai-yeon.
North Korea is believed to be developing nuclear weapons - the United States has said in the past it may well have one or two already - but experts say any test would have been easily detected.
North Korea has said nothing so far about a blast, but often reports on events long after they happen.