SEOUL, South Korea -- An explosion that shot a 2-mile-wide mushroom cloud into the sky was the planned demolition of a mountain for a hydroelectric project, North Korea said Monday, and it invited a British diplomat to visit the site.
Experts from the United States and elsewhere say they don't believe Thursday's blast near the Chinese border was a nuclear test.
A Bush administration official said the United States has indications the North is trying to conduct a test. The explosion and concerns over Pyongyang's nuclear ambitions set off a heated back-and-forth between the White House and Democratic presidential challenger John Kerry.
North Korea denounced the speculation over a nuclear test as part of a "preposterous smear campaign" aimed at diverting world attention away from revelations about past South Korean nuclear activities, Pyongyang's official KCNA news agency said.
Speaking on the condition of anonymity, a U.S. official said it isn't clear what happened. While the official said there isn't any reason to believe it was a nuclear test, the official also couldn't confirm the North Koreans' explanation that it was linked to construction of a hydroelectric project.
A U.S. defense official said officials have seen none of the telltale signs of a nuclear detonation -- radiation, seismic disturbances and human casualties -- an indication that the blast was likely conventional.
The American military operates satellites and other sensors capable of distinguishing a nuclear detonation from a conventional explosion.
A U.N. official said the Comprehensive Nuclear-Test-Ban Treaty Organization, which monitors nuclear activity, had not picked any signs that the explosion was a nuclear blast.
The North's official news agency KCNA said "blastings at construction sites of hydro-power stations in the north of Korea" had taken place.
North Korean Foreign Minister Paek Nam Sun told the same to visiting British Foreign Office minister Bill Rammell.
In an interview with the British Broadcasting Corp., Rammell said Paek told him the blast "wasn't an accident, that it wasn't a nuclear explosion, that it was a deliberate detonation of a mountain as part of a hydroelectric project."
Rammell said the North Koreans "have nothing to fear and nothing to hide and should welcome the international community actually verifying the situation for themselves."
North Korea told Britain's ambassador in Pyongyang, David Slinn, that he can visit the blast site as soon as Tuesday to verify its claims, the Press Association of Britain reported.
South Korean Unification Minister Chung Dong-young said his country would look into whether the site "is an area for constructing a hydroelectric power plant," according to the news agency Yonhap.
Andrew Kennedy, head of the Asia program at the Royal United Services Institute in London, said the North Korean explanation has "a ring of truth to it" and that if diplomats were allowed to take a Geiger counter, they would easily know whether a nuclear blast occurred.
"The North Koreans would know that with the intelligence and the surveillance satellites that the West has, it would be very easy to check. That is backed up by the North's agreement to allow the visiting British diplomat to go to the site and inspect it," he said.
"North Korea is usually trying to convince people that they do have a nuclear capability. ... It's not in their interest to keep a nuclear test quiet," he added.
There was no comment from the International Atomic Energy Agency, whose inspectors were told to leave North Korea after it quit the Nonproliferation Treaty last year.
The size of the reported explosion on the 56th anniversary of the founding of North Korea had raised speculation that it might be a nuclear test. Secretary of State Colin Powell said Sunday there was no indication the blast was from a test.
Kerry said just the idea that the United States was thinking North Korea might test a nuclear weapon highlights a national security failure by President Bush. Under Bush's watch, North Korea has advanced its nuclear program, Kerry said.
"North Korea's nuclear program is well ahead of what Saddam Hussein was even suspected of doing -- yet the president took his eye off the ball, wrongly ignoring this growing danger," Kerry said in a statement. "What is unfolding in North Korea is exactly the kind of disaster that it is an American president's solemn duty to prevent."
In a telephone call Sunday to The New York Times, Kerry accused the administration of letting "a nuclear nightmare" develop by refusing to deal with North Korea.
Bush spokesman Scott McClellan accused Kerry of wanting to return to "the failed Clinton administration policy" on North Korea. He said that while Clinton's 1994 agreement with North Korea calling for a freeze fell apart, Bush is trying to rally North Korea's neighbors to pressure the country to abandon its nuclear activities.
IAEA chief Mohamed ElBaradei on Monday chided South Korea, expressing "strong concern" that Seoul had not informed the agency of its nuclear activities. He revealed that Seoul produced more than 300 pounds of uranium metal in the 1980s.
It then used some of that metal in nuclear enrichment experiments using laser technology conducted in 2000.
Diplomats said the use of the metals developed earlier raised doubts over Seoul's explanation that the 2000 experiments were carried out by renegade scientists.