A defiant North Korea has conducted its third nuclear test, prompting a wave of international criticism from governments and other organization. It also said that more “measures” may follow, raising concerns that more nuclear devices may be exploded.

Track LIVE UPDATES on the fallout of the North Korean nuclear test.

Pyongyang said the Tuesday morning explosion was part of an effort to protect its national security and sovereignty, citing US opposition to the recent North Korean space launch.

"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," North Korea's KCNA state news agency said.

The move came in defiance of the UN and individual nations, which have pressured North Korea not to proceed with its plan. After the test sparked condemnation, Pyongyang threatened that if the US responds to the test “with hostility,”then unspecified “second and third measures” may follow. This corresponds with earlier speculation that Pyongyang seeks to detonate more than one nuclear device.

North Korean diplomat Jon Yong Ryong also told the UN disarmament forum in Geneva that his country "will never bow down to any resolution," in respondr to criticisms that the nuclear test violated several UN Security Council resolutions banning such actions.

South Korean President-elect Park Geun-hye strongly condemned the new test. She said her incoming administration would not tolerate a nuclear-armed North Korea “under any circumstances,” and pledged to enact strong deterrence measures against Pyongyang's nuclear program.

The Chinese Foreign Ministry has urged all parties involved to reduce tensions and solve the issue through dialogue in the framework of six-party talks. It also expressed “firm opposition” to the test, called on North Korea not to take any actions that would aggravate the situation, and to “honor its commitment to denuclearization”.

South Korean passengers watch TV news reporting North Korea's apparent nuclear test, at the Seoul train station on February 12, 2013. (AFP Photo/Kim Jae-Hwan)

US President Barack Obama warned that both Tuesday’s test and the earlier satellite launch are provocations, and that “far from achieving its stated goal of becoming a strong and prosperous nation, North Korea has instead increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction and their means of delivery.” He threatened"further swift and credible action" against Pyongyang.

UN chief Ban Ki-moon has condemned the nuclear test, calling it “deplorable” and a “grave violation of the relevant Security Council resolutions.” The statement released by Ban’s spokesperson voiced concern over the “negative impact of this deeply destabilizing act on regional stability as well as the global efforts for nuclear non-proliferation.”

The test was also criticized by Britain, Russia, Japan, France, Germany, the EU, the IAEA and NATO.

South Korea called for an emergency meeting of the United Nations Security Council to discuss the developments. It is expected to be held at 9:00 a.m. EST (1400 GMT) on Tuesday.

Protesters shout slogans and raise their fists towards the headquarters of the pro-Pyongyang General Association of Korean Residents in Japan (Chongryon) as police officers stand guard in Tokyo February 12, 2013, after North Korea conducted its third nuclear test. (Reuters/Toru Hanai)

The likely response to the nuclear test will be a new round of sanctions from the UN. But no matter how many sanctions other nations impose on Pyongyang, it is unlikely to yield to demands voiced by Washington, Asia specialist Tim Beal explained.

“No country really changes policy under sanctions if the alternative, what is being required, is worse than the sanctions,” he told RT. “And that is the case with North Korea. North Korea in a sense could surrender to American demands, but that in fact in their eyes would be worse that what the Americans can do to them with sanctions. So they will persevere until the Americans come to the negotiation table.”

The United States Geological Survey confirmes an earthquake in North Korea's northeast of between 4.9- and 5.1-magnitude, at a depth of about one kilometer.

The Japanese Meteorological Agency reports that the tremor's epicenter was located in Kilju county, at exactly the same place and depth as the quake caused by North Korea's last known underground nuclear test in 2009. North Korea’s first nuclear test in 2006 was also carried out at the Punggye-ri test site.

Japan Meteorological Agency's earthquake and tsunami observations division director Akira Nagai points at a graph of ground motion waveform data observed in the morning in Japan during a news conference in Tokyo February 12, 2013. (Reuters/Toru Hanai)

Pyongyang informed the US and China of its plans for a nuclear test on Monday, Yonhap reported. North Korea said it would continue with the test despite pressure from the UN Security Council and its non-UNSC neighbors.

The South Korean military estimate that the yield of the nuclear explosion was between six and seven kilotons. Russia’s defense ministry says the size of the blast was over seven kilotons. The Vienna-based Comprehensive Nuclear-Test-Ban Treaty Organization said the yield was roughly twice as big as the previous nuclear test in 2009.

Further investigation into the nature of the explosion is underway. The evidence gathered – including seismic data, satellite images and data from spy planes detecting radioactive fallout – could allow researchers to deduct the status of North Korea’s secretive nuclear program. So far, the isolated country was believed to be unable to build a nuclear device small enough to fit onto one of its long-range ballistic missiles, making its nuclear capabilities virtually useless for offensive warfare.

Concerns over the claimed miniaturization effort were fueled by North Korea's rocket launch last December. Pyongyang said it put a satellite into orbit for civilian purposes, and for national prestige, but many countries claimed it was a clandestine rocket weapons test. The UN Security Council condemned the launch, which it said was carried out in violation of a UNSC resolution banning the development of ballistic technology by North Korea.

Japan's Prime Minister Shinzo Abe (C) speaks to media after attending a meeting of Security Council of Japan at his official residence in Tokyo February 12, 2013 after reports of North Korea's possible nuclear test. (Reuters/Issei Kato)

An hour after the test, Japan said that it is considering leveling further sanctions against North Korea.

"I have ordered that we consider every possible way to address this issue, including our own sanctions, while cooperating with other countries," Japanese Prime Minister Shinzo Abe told reporters after a meeting of Japan's security council.

The news of the suspicious seismic activity in North Korea came days after South Korea and the US threatened that they may carry out a pre-emptive strike at North Korean facilities to halt its nuclear program.

China, North Korea’s main economic partner and only ally, said Pyongyang would pay a “heavy price” and threatened to scale down aid should it carry out a nuclear test.

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