According to the South Korean news agency Yonhap, the cloud, more than two miles in breadth, was due to a massive explosion, far stronger than an April explosion that killed 160 people and injured an estimated 1,300 at a North Korean railway station after a train carrying oil and chemicals apparently hit power lines.

3,000 hurt in N.Korea train blast

Last week's explosion occurred in North Korea's northern inland province of Ryanggang on Thursday, the 56th anniversary of the foundation of North Korea. While admitting that the cause was a mystery, South Korean and US officials said it was unlikely that it was caused by a nuclear explosion.

''We don't think, at this point, it was a nuclear event, but we're looking at it and will get further analysis,'' US national security adviser Condoleezza Rice told CNN Sunday.

US intelligence agencies, however, had been reporting for sometime that North Korea was close to testing a nuclear device.

Khan saw North Korean nukes  

On Saturday, Pyongyang accused South Korea of conducting secret nuclear experiments, which would spur North Korea's nuclear programme.

On Sunday, Democratic presidential contender John Kerry released a statement decrying the Bush Administration's position on North Korea.

The Pakistan North Korea nexus 

'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,' he said. 'What is unfolding in North Korea is exactly the kind of disaster that it is an American president's solemn duty to prevent.'