The American government has said it does not believe that a large explosion in North Korea was a nuclear test.
The explosion took place in a remote region near a secret military base.
It was filmed by satellites last Thursday, as the Stalinist state celebrated its National Day, but news of it was only made public this morning.
The incident has fuelled speculation that North Korea may be close to its first test of a nuclear bomb.
The US National Security Advisor Condeleeza Rice said it would be a mistake for North Korea to make such a move.
