The United States and South Korea have played down suggestions that a massive explosion in North Korea last week was caused by a nuclear device.
North Korea keeps a tight grip on information
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.
The blast created what officials in Seoul say was a peculiar-shaped cloud.
As the secretive North stays silent on the blast, a visiting UK minister has asked for an urgent explanation.
An unnamed diplomatic source, who spoke to South Korean news agency Yonhap in Beijing, said a cloud with a radius of up to 4km (2.5 miles) had been spotted in Yanggang's Kimhyungjik County.
The area is mountainous and thinly populated, and home to an underground military base known to contain medium-range missiles.
There has been no comment on the incident so far from either North Korea or its closest ally, China.
In April, an explosion at a railway station in North Korea killed more than 150 people - but Pyongyang only admitted the incident three days later.
'Not a test'
US Secretary of State Colin Powell said he did not believe the blast was connected to North Korea's nuclear programme.
"There was no indication that was a nuclear event of any kind. Exactly what it was, we're not sure," Mr Powell told ABC television on Sunday.
The BBC's Charles Scanlon, in Seoul, said the South Korean government had confirmed the explosion but was still assessing satellite images of the blast.
The South Korean president's office told AFP news agency: "Our government information for now shows North Korea has not conducted any nuclear test.
"We are trying to confirm whether it is fireworks, a fire in
mountains or an accidental explosion."
It was unlikely that North Korea would have carried out a nuclear test so close to the border with China, our correspondent says.
He adds that the US suspects that the closed region where the blast occurred may be where the North is conducting an alleged uranium enrichment programme.
UK Foreign Office Minister Bill Rammell has demanded an immediate explanation from the North's foreign ministry.
Mr Rammell told the BBC he would also be pressing the North to rejoin the "six-party" international talks - involving both Koreas, the US, China, Russia and Japan.
"I think this is going to be a long haul and it's about trying to pull North Korea back from complete isolation," he added.
The authorities there have been under huge international pressure to end all nuclear programmes and disarm, but have offered only limited concessions in the three rounds of six-party talks to date.
They have not so far committed to attending a fourth round, which the Chinese, as hosts, wanted to start before the end of this month.
On Saturday, the North's foreign ministry said reports of nuclear experiments in the South made it even more determined to pursue its own programme.