SEOUL -- 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 yesterday, and it invited a British diplomat to visit the site.
Specialists from the United States and elsewhere say they do not 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 nominee John F. 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 US official said it is not clear what happened. While the official said there is no reason to believe it was a nuclear test, the official also could not confirm the North Koreans' explanation that it was linked to construction of a hydroelectric project.
A UN 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.
KCNA, the North's official news agency, said ''blastings at construction sites of hydropower stations in the north of Korea" had taken place.
Foreign Minister Paek Nam Sun of North Korea told the same to visiting minister Bill Rammell of the Foreign and Commonwealth Office in Britain.
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 today to verify its claims, the Press Association of Britain reported.
Unification Minister Chung Dong-young of South Korea said his country would look into whether the site ''is an area for constructing a hydroelectric power plant," according to the news agency Yonhap.
There was no comment from the International Atomic Energy Agency, whose inspectors were told to leave North Korea after it quit the Nuclear 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 L. 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."
A 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.