No, it was not a nukular explosion as previously speculated here and elsewhere. The
BBC reports that DPRK Foreign minister said the explosion was part of an hydro-electric development in the northern part of the country.
The country's foreign minister, Paek Nam Sun, said the blast was the deliberate demolition of a mountain as part of a hydro-electric project.
Update [2004-9-13 8:12:49 by ClaudeB]:According to a
new story by the BBC, UK Foreign Office Minister, Bill Rammell, who is presently in North Korea for talks with senior Pyongyang officials, has obtained a permission for foreign diplomats to visit the site of the explosion.