A huge explosion rocked North Korea last week but US and South Korean officials said on Sunday it was unlikely to have been a nuclear weapons test despite the appearance of a "peculiar cloud" over the area.
South Korea first got indications of the blast from a satellite, but it was too early to say whether it was a bomb, a senior South Korean official told Reuters. It probably occurred between Wednesday evening and Thursday morning.
"The weather overall at the time was cloudy but there was a peculiar cloud, a cloud that was different from any other," said the official, who asked not to be identified. "We cannot confirm whether it had the characteristics of a mushroom cloud."
Secretary of State Colin Powell said the United States did not believe North Korea had conducted a nuclear test.
"They haven't conducted tests to the best of our knowledge and belief, and the activity reported today is not conclusive that they are getting ready to do one or not," he told NBC TV.
South Korea's Yonhap news agency said the blast in Kimhyungjik county in Ryanggang province in the remote north-east near the border with China appeared much stronger than a train explosion that killed at least 170 people in April.
A mushroom cloud up to 4 km (2.5 miles) in diameter was seen after the blast in an area near missile bases, Yonhap said.
South Korea's Chosun Ilbo newspaper said the site was 10 km (6 miles) south-west of the Yongjori Missile Base at a point 30 km (18 miles) from the China frontier. The mountainous area is off-limits to outsiders, including aid workers.
The base has tunnels for storing, deploying and launching medium-range Rodong missiles, according to defector reports.
The North has said nothing yet about a blast but often notes events long after they happen - and sometimes not at all.
A nuclear test would radically alter the stakes in its stand-off with Washington over Pyongyang's atomic ambitions.
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.
British Foreign Office Minister Bill Rammell, visiting North Korea, told BBC radio he had asked for an urgent explanation.
"I have said that I am going to be raising it with the (Korean) foreign minister when we meet tomorrow," he said.
CHINESE VISIT: North Korea is believed to be developing nuclear weapons - Washington has said it may have one or two or even more already - but experts say any test would have been detected easily.
John Bellini, a geophysicist at the US Geological Survey in Golden, Colorado, said his organisation had not detected an explosion large enough to be considered nuclear.
"If it was a nuclear bomb, that would have been detected by everybody in the world," he said. A blast involving a conventional bomb would have been too small to detect, he said.
A senior Chinese Communist Party delegation met leader Kim Jong-il on Sunday, the North's official KCNA news agency said. It said the Chinese handed Kim a letter from President Hu Jintao and promised aid for the impoverished country.