North Korea's official KCNA news agency confirmed on Tuesday that the country had successfully conducted a third nuclear test.
"It was confirmed that the nuclear test that was carried out at a high level in a safe and perfect manner using a miniaturized and lighter nuclear device with greater explosive force than previously did not pose any negative impact on the surrounding ecological environment," KCNA said.
The isolated country is banned under United Nations Security Council resolutions from developing nuclear and missile technology following previous tests in 2006 and 2009.
Following Tuesday's test, South Korea's mission to the UN called an emergency Security Council meeting to be held in New York later in the day. South Korea holds the presidency of the 15-nation council this month.
The European Union's foreign policy coordinator, Catherine Ashton condemned the test.
"This nuclear test is a further blatant challenge to the global non-proliferation regime and an outright violation of the [North Korea's] international obligations not to produce or test nuclear weapons," she said in a statement.
German Foreign Minister Guido Westerwelle condemned the test "in the strongest possible terms." a statement released by Westerwelle's office said. "Further sanctions against the regime in Pyongyang should now be considered."
UN Secretary-General Ban Ki-moon condemned the nuclear test as "deplorable."
"The secretary-general condemns the underground nuclear weapon test conducted by [North Korea] today," Ban's spokesman, Martin Nesirky, said in a statement. "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," he added.
Japanese Prime Minister Shinzo Abe said the test must not be tolerated.
"It is a grave threat to our nation's safety and cannot be tolerated as it will significantly damage international society's peace and safety," Abe said.
US President Barack Obama described North Korea's nuclear test as a "highly provocative act.”
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community. The United States will also continue to take steps necessary to defend ourselves and our allies," a White House statement said.
North Korea's communist neighbor and closest ally, China, expressed "firm opposition" to the nuclear test.
"We strongly urge the DPRK to honor its commitment to denuclearization, and not to take any actions which might worsen the situation," the foreign ministry said in a statement posted on its website.
International monitoring agencies detected an earthquake with a magnitude of between 4.9 and 5.2 at 10:57 a.m. local time (01:57 GMT) prompting initial speculation that the tremor was the result of a nuclear test. The quake's epicenter, which was only one kilometer ( 0.6 of a mile) deep, appeared to be near the site of previous nuclear tests.
The head of the UN's Nuclear-Test-Ban Treaty Organization, Tibor Toth, said in a statement that the tremor showed "clear explosion-like characteristics." Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.
North Korea vowed last month to conduct "high-level" nuclear tests in retaliation for the tightening of Security Council sanctions following the country's test of a long-range rocket last December.
hc/pfd (Reuters, AP, AFP, dpa)