In the first remarks to the outside world concerning last Thursday's mysterious explosion, North Korea reportedly has said the blast was part of a construction project. Other countries, however, have been unable to determine what caused the blast.

North Korean foreign ministry officials have told British and Chinese diplomats that the explosion was the demolition of a mountain to make way for a hydro-electric plant.

Chinese and British journalists reported the news.

North Korea had remained silent about the blast last Thursday, fueling speculation about its cause.

Initial reports spoke of a mushroom cloud several kilometers in diameter, prompting concern that North Korea might have conducted its first nuclear weapons test.

Japanese government spokesman Hiroyuki Hosoda on Monday said while the real cause is still a mystery, it is almost certain the explosion was not a nuclear detonation. Mr. Hosoda says no significant seismic waves were recorded, which usually would be a tell-tale sign of a nuclear test.

U.S. and South Korean officials also are playing down speculation that the blast was related to North Korea's nuclear weapons programs.

Intelligence satellites spotted the blast in Ryanggang Province, near the Chinese border. The area is home to a facility believed to hold launch facilities for medium-range missiles. The United States, along with China, South Korea, Japan and Russia, is attempting to persuade North Korea to abandon its nuclear programs. In recent weeks, doubts have risen that Pyongyang will attend a fourth round of talks on the issue expected later this month.

Tensions escalated nearly two years ago when U.S. officials said North Korea had admitted having a uranium-based weapons program, in violation of a 1994 agreement.

Pyongyang has denied any such program but claims to be reprocessing 8,000 spent fuel rods for plutonium-based weapons.

There has been increasing concern recently in Washington that North Korea might be preparing for a nuclear weapons test.

The White House national security advisor, Condoleezza Rice on Sunday, warned Pyongyang that it would be mistaken if it thought such a test would give it negotiating leverage. She said a test would isolate the communist state even further.