Washington and Seoul have said the explosion was unlikely to have been a nuclear weapons test. South Korean media said an accident at an underground munitions depot or a weapons factory was a likely explanation for possibly two blasts.
South Korea's financial markets, which can react sharply to developments in the North, ignored the blast reports, which came as diplomats were seeking to persuade Pyongyang to return this month to six-party talks on its nuclear weapons programs.
"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 with British Foreign Office minister Bill Rammell quoted North Korean Foreign Minister Paek Nam-sun as saying.
Britain's Press Association gave similar details in a pool report, and China's Xinhua news agency quoted a North Korean Foreign Ministry official as giving the same explanation.
Paek, who was providing 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.
The BBC said when Paek was asked why North Korea had not explained earlier about the blasts, he told Rammell Pyongyang had not done so because all foreign journalists were liars.
Rammell was the highest British official to visit Pyongyang, and he had been expected to meet the North's leader Kim Jong-il. He also asked Paek to allow diplomats to visit the blast site.
"He has said he will consider that request and get back to me," Rammell said.
South Korean officials confirmed indications of one large explosion — or possibly two — on the night of Sept. 8 and early morning of Sept. 9.
South Korean Unification Minister Chung Dong-young told a parliamentary committee on Monday "a peculiar cloud" and seismic activities were detected in separate areas that may be from unrelated incidents some 100 to 120 km (60 to 75 miles) apart.
Missile Bases in Area
South Korean media did not home in on the possibility of controlled explosions for building work.
South Korean intelligence detected two explosions on the night of Sept. 8 to 9, the JoongAng Ilbo newspaper in South Korea said, suggesting a possible accident at a military factory or a munitions depot.
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 Defense 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."
A train blast in April in North Korea's Ryongchon rail station that killed at least 170 people was believed to have been sparked by careless handling of explosive materials.
Analyst Kim Kyung-sool at Korea Energy Economics Institute said Ryanggang offers rich resources for hydroelectric power, with several smaller plants in place, but there had been no reports of a larger-scale project.
The North's state television last week showed footage of what it said was the construction site for a hydroelectric power plant about 60 km (40 miles) east of the reported explosion. The video showed earthmovers with rugged mountains in the background.
U.S. officials, including Secretary of State Colin Powell, played down the possibility of a nuclear test.
North Korea is believed to be developing nuclear weapons; Washington has said it may have one or two or even more already.
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.
The news broke as South Korea, Japan, China, Russia, and the United States sought to persuade Pyongyang to resume talks on its nuclear ambitions. The North, which threatened at earlier talks to test an atomic bomb, says it sees no need for more talks.
Additional reporting by Park Sung-woo, Lee Shin-hyung, Frances Yoon, and Lee Jun-goo in Seoul and Marie Frail in Beijing