The cause of a massive cloud which appeared over North Korea last week remains shrouded in mystery.
Satellite images from before (l) and after (r) the blast show little change
North Korea says controlled explosions related to dam building caused the cloud, which South Korean observers described as mushroom shaped.
But a South Korean satellite which photographed the area on Wednesday provided inconclusive evidence.
And South Korea's National Intelligence Service said the cloud may in fact have been a natural formation.
"There might have been a blast to build a hydroelectric power
dam... or there might have been natural clouds with a peculiar (mushroom-like) shape," the agency was quoted as reporting.
The confusion may be resolved on Thursday, when Britain's ambassador in Pyongyang, David Slinn, and a group of other foreign diplomats are due to visit the site and seek further information.
The cloud, which appeared near Yongjo-ri in Yanggang Province, initially triggered fears North Korea had risked the wrath of the international community by testing a nuclear device.
The US and South Korea have since discounted that possibility.
US Secretary of State Colin Powell said North Korea's explanation for the blast squared with the information Washington had gathered.
"The information they gave is consistent with what we saw,
that it might have been demolition work for a hydroelectric
facility," Mr Powell told Reuters news agency on Tuesday.