"It was no nuclear explosion or an accident. It was a deliberate controlled detonation to demolish a mountain in the far north of the country," a BBC correspondent in Pyongyang quoted North Korean Foreign Minister Paek Nam-sun as saying.
Paek, who was giving the first North Korean word on the explosion, said it was part of a construction project to build a hydro-electric dam in the remote mountainous region of Ryanggang on the Chinese border.
When asked why Pyongyang had not explained earlier about the blasts, he reportedly told British Foreign Office Minister Bill Rammell because all foreign journalists were liars.
On Sunday, South Korean officials confirmed indications of one large explosion - or possibly two - on the night of 8 September and the early morning of 9 September.
Rammell is the highest British official to visit Pyongyang and he had been expected to meet leader Kim Jong-il. He also asked Paek to allow a visit to the blast site.
"He has said he will consider that request and get back to me," Rammell said.
South Korean intelligence detected two explosions on the night of 8 to 9 September, the JoongAng Ilbo newspaper in South Korea said, suggesting a possible accident at a military factory or a munitions depot.
South Korean officials are
analysing satellite images
The area in Ryanggang that borders China has been known to have a secret missile base, but the site of the accident is some distance away, the source was quoted as saying.
Analyst Kim Tae-woo at the Korea Institute for Defence Analysis said it was probably an accident.
"Rodong missile bases are located in the blast area, and extremely explosive liquid fuel which is also very flammable is used in production and in operation of missiles," Kim said. "I believe it was a mere accident triggered by mishandling."
South Korean officials are analysing satellite images and other data to determine the cause of the explosion that produced a large cloud detected on 9 September.
US officials, including Secretary of State Colin Powell, also played down the possibility of a nuclear test.
North Korea is believed to be developing nuclear arms - Washington has said it might have one, two or even more nuclear weapons.
In October 2002, US officials said North Korean officials had admitted they had a clandestine uranium enrichment programme that could be used to develop nuclear weapons and that violated its international commitments. They subsequently denied this.