"Japan firmly protests and sternly condemns the nuclear testing."

Statement from the office of Ban Ki-moon, U.N. secretary-general:

"The Secretary-General condemns the underground nuclear weapon test conducted by the Democratic People's Republic of Korea (DPRK) today. It is a clear and grave violation of the relevant Security Council resolutions.

It is deplorable that Pyongyang defied the strong and unequivocal call from the international community to refrain from any further provocative measures. The Secretary-General had repeatedly called on the new leadership in Pyongyang to address international concerns and start building confidence with neighboring countries and the international community.

The Secretary-General is gravely concerned about the negative impact of this deeply destabilizing act on regional stability as well as the global efforts for nuclear non-proliferation. He once again urges the DPRK to reverse course and work toward de-nuclearization of the Korean peninsula.

The Secretary-General is confident that the Security Council will remain united and take appropriate action. In the meantime, the Secretary-General remains in close contact with all concerned parties and stands ready to assist their efforts."

Yukiya Amano, International Atomic Energy Agency director general:

"This is deeply regrettable and is in clear violation of U.N. Security Council resolutions."

"The IAEA remains ready to contribute to the peaceful resolution of the DPRK nuclear issue by resuming its nuclear verification activities in the country as soon as the political agreement is reached among countries concerned."

Statement from NATO:

"We condemn in the strongest terms the test by North Korea of a nuclear weapon, conducted in flagrant violation of relevant United Nations Security Council resolutions.

This irresponsible act, along with the December missile launch, poses a grave threat to international and regional peace, security and stability. North Korea's pursuit of weapons of mass destruction represents continued defiance of the U.N. Security Council and the broader international community."

William Hague, British foreign secretary:

"I strongly condemn this development, which is a violation of U.N. Security Council Resolutions 1718, 1874 and 2087. North Korea's development of its nuclear and ballistic missile capabilities poses a threat to international and regional security. Its repeated provocations only serve to increase regional tension, and hinder the prospects for lasting peace on the Korean Peninsula."

Francois Hollande, French president:

"I condemn in the strongest terms the nuclear test just made by North Korea," Hollande said on the French government's website.

"France again urges North Korea to immediately comply with its international obligations and to carry out complete, verifiable and irreversible dismantling of its nuclear and missile programs."

Statement from the German Foreign Office:

Foreign Minister Guido Westerwelle "strongly condemns" North Korea's nuclear test. The "international community should consider further sanctions against the regime," the agency's official Twitter feed said.

Bob Carr, Australian foreign affairs minister: