North Korea's test - failure or fake? This is strange:

Within a few hours, Russia said it was "100% certain" a nuclear test had been carried out and measured it at between five to 15 kilotons.

China also "has not given any indication it doubts Pyongyang's claim about the nuclear test". But everybody else is highly dubious.

South Korea received a sophisticated radioactivity detector from Sweden on Wednesday to carry out further tests, officials said.

And US and Japanese monitoring planes based on the island of Okinawa are also believed to be flying in the region.

But officials have warned that the process could take time, and South Korea has warned it could be two weeks before conclusive proof is available.

Others, including one French nuclear official, have warned that Monday's nuclear test may never be confirmed.

Xavier Clement of France's Atomic Energy Commission said the seismic data had to be sifted to differentiate the blast from background noise of subterranean movement.

"It is possible that this cannot be done, given the weakness of the signals compared to the background noise," he told the French news agency AFP.

Kim is threatening a second test, which could clear up any doubts.