The United Nations nuclear watchdog has North Korea's nuclear programme a "grave concern" after the nation, to which the agency's experts have no access, conducted its sixth nuclear test since 2006.

"Today’s nuclear test by the Democratic People's Republic of Korea is an extremely regrettable act... Once again, I strongly urge the DPRK to fully implement all relevant resolutions," Yukiya Amano, the head of the International Atomic Energy Agency, said in a statement.

"The agency continues to closely follow developments in the DPRKs nuclear programme, which is a matter of grave concern."

North Korea said the test was a successful detonation of an advanced hydrogen bomb rather than a standard nuclear fission device.

The Comprehensive Test Ban Treaty Organisation (CTBTO), which monitors seismic and radionuclide data worldwide, said the explosion was stronger than previous ones and was located at the site of earlier nuclear tests.

"If confirmed as a nuclear test, this act would indicate that the DPRK's nuclear programme is advancing rapidly," the CTBT's Secretary General Lassina Zerbo said.

The CTBT bans all nuclear explosions but it will only enter into force if all countries with advanced nuclear technology ratify it.

The outstanding nations are China, Egypt, North Korea, India, Iran, Israel, Pakistan and the United States.

"I sincerely hope that this will serve as the final wake-up call to the international community to outlaw all nuclear testing by bringing the CTBT into force," Mr Zerbo said.