Earlier, U.S. and South Korean officials sought to allay fears that a nuclear weapon had been tested.
According to a BBC report, a visiting British cabinet minister pressed his hosts on the issue, and North Korean Foreign Minister Paek Nam-sun said the explosion had been the deliberate demolition of a section of mountain for a hydro-electric project.
The British minister, Bill Rammell, was reported as saying he had asked for diplomats to be allowed to visit the scene to see for themselves.
South Korea's Yonhap news agency reported Sunday that an explosion which sent up a cloud of smoke more than two miles across had taken place last Thursday, the 56th anniversary of the founding of North Korea and an important day in the reclusive state.
The incident, which it said was first detected by a South Korean satellite, occurred in the northernmost Ryanggang province, in an area close to the border with China.
The reports brought new tensions to a region already uneasy about North Korea's ambitions to become a nuclear weapons state. It also raised suspicions that Kim Jong-il may be trying to bring additional pressure to bear on the Bush administration during the election campaign.
Locked in a standoff with the U.S. over its nuclear programs since October 2002, Pyongyang has on several occasions threatened to test a nuclear device.
Secretary of State Colin Powell told the ABC network Sunday there was no indication that a nuclear event had taken place.
Powell confirmed reports that there had been some unusual activity recently at North Korea's atomic sites but said it was "not conclusive that they're moving toward a test."
South Korean presidency spokesman Kim Jong-min said the nature of the explosion had not yet been determined.
On Monday, Yonhap quoted diplomatic sources as saying South Korean and Chinese officials were trying to make contact with people who had witnessed the blast - specifically North Korean businessmen who were crossing over into China near the area of the reported explosion.
South Korean papers reported the theory that an accident at an underground munitions factory or depot may have been responsible.
China, North Korea's closest ally and host of several rounds of multi-party talks aimed at resolving the North Korean nuclear issue, did not comment officially on the blast report.
Last April, a huge explosion occurred at a North Korean railway station, reportedly killing some 160 people and wounding many more. Pyongyang, which for three days did not confirm the incident had even taken place, said later that a train carrying chemicals had hit power lines.
According to sources cited by Yonhap, last Thursday's explosion was considerably bigger than the blast last April.
South Korean officials were quoted as saying Thursday's explosion site in Ryanggang's Kimhyungjik county was in a mountainous area near a key intermediate-range Taepo Dong missile base.
South Korean media in July 1999 cited government officials as saying North Korea was building a new missile base in Ryanggang with 10 missile launch towers, strategically located to make it difficult to attack with precision munitions or cruise missiles.
North Korea first test-fired a Taepo Dong - designed to carry a nuclear warhead - in 1998, unnerving the region by lobbing the rocket over Japan and into the Pacific Ocean. South Korean and Western intelligence agencies say it is pressing ahead with advanced varieties with considerably more ambitious ranges, threatening Alaska and U.S. forces in the Pacific.
Many Asian governments worry that if North Korea does become a declared nuclear power by successfully testing a weapon, other countries in the region may decide to pursue similar programs themselves, setting off a nuclear arms race.
South Korea, Japan and Taiwan have all displayed interest in a nuclear deterrent past decades and have advanced civilian nuclear programs.
In just the past couple of weeks, South Korea has admitted that its scientists carried out uranium enrichment and plutonium experiments respectively in 2000 and 1982.
Seoul insisted the unauthorized activity was prompted by academic interest and was not aimed at developing a nuclear weapon capability, but North Korea said at the weekend its neighbor's admissions made it even more determined to pursue nuclear programs of its own.
Meanwhile, attempts to arrange another round of six-party talks in Beijing this month are running into difficulties, with North Korea raising doubts that it will attend.
A flurry of diplomatic activity is underway, with U.S. envoy James Kelly visiting Beijing, and British and Chinese officials traveling to North Korea for talks.
The six parties involved in previous meetings are the U.S., both Koreas, Japan, China and Russia.
Three previous rounds have failed to find a breakthrough.
At the last six-way talks, held in Beijing last June, Washington put forward a proposal that would bring North Korea "provisional" security guarantees from the U.S. and energy aid from other countries, in return for a commitment to freeze its nuclear programs.
Those temporary measures would only become permanent once the programs were completely dismantled, and North Korea would have just three months to freeze its facilities. It would also have to allow full verification of the process.
North Korea has responded negatively to the proposal, and continues to deny the existence of a clandestine uranium-enrichment program. It only admits to a separate, plutonium-based program.
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