North Korea claims to have conducted its fifth, and by far most powerful, nuclear bomb test on Friday morning, in defiance of United Nations resolutions. Condemnation from world leaders was immediate, including even North Korea’s patron China.

It remains possible that the event in question was an earthquake, but there is growing international confidence that it was indeed an underground nuclear test. The seismic activity occurred near the Punggye-ri nuclear test site. CNN reports satellite images have shown increased activity at the site in recent weeks.

“The US Air Force is expected to start flying the WC-135 Constant Phoenix Aircraft to take air samples and see if it can determine a nuclear event occurred. Japan has sent four jets to test for radiation,” CNN added.

More dubiously, the North Koreans claim the device they tested either can be mounted on ballistic missiles or puts them much closer to developing such technology. Pyongyang claimed it had produced ICBM-compatible warheads six months ago, but the claim was widely perceived as false. Their claim of detonating a more advanced hydrogen bomb has also been broadly disputed.

CNN quotes a South Korean lawmaker, Kim Byung-kee of the opposition Minjoo Party, who worried that North Korea is approaching nuclear-missile capability “faster than previously expected,” although he said he doubted they already have such capability or will develop it within the next year or two.

North Korea has been energetically testing banned ballistic-missile technology lately, to the great consternation of Japan, which has reported North Korean test missiles splashing down within 300 kilometers of its coastline. It is possible Pyongyang wants to combine these nuclear bomb and missile tests to give the impression they are on the verge of launching a nuclear missile, or simply to cause maximum alarm in the world community.

They certainly seem to have accomplished that goal. “The South accused the North’s leader Kim Jong-un of ‘maniacal recklessness.’ China ‘firmly opposed’ the test, Japan ‘protested adamantly’ and the US warned of ‘serious consequences’ including ‘new sanctions.’ The UN Security Council will meet later behind closed doors to discuss the issue,” the BBC reports.

The Russians condemned the new nuclear test as well, insisting that North Korea “stop its dangerous escapades and unconditionally implement all resolutions of the United Nations Security Council.”

U.N. Secretary-General Ban Ki-moon, who hails from South Korea, denounced the North’s actions as a “brazen breach” of U.N. resolutions and called on the Security Council to “unite and take urgent actions,” according to the New York Times.

“This unacceptable act endangers peace and security in the region and is another vivid reminder of the urgent need to strengthen the global nuclear test ban regime,” said Ban.

“The United States condemns North Korea’s September 9 nuclear test in the strongest possible terms as a grave threat to regional security and to international peace and stability,” said President Obama in his statement:

North Korea stands out as the only country to have tested nuclear weapons this century. Today’s test, North Korea’s second this year, follows an unprecedented campaign of ballistic missile launches, which North Korea claims are intended to serve as delivery vehicles for nuclear weapons targeting the United States and our allies, the Republic of Korea and Japan.”

“As Commander in Chief, I have a responsibility to safeguard the American people and ensure that the United States is leading the international community in responding to this threat and North Korea’s other provocations with commensurate resolve and condemnation,” Obama continued:

To be clear, the United States does not, and never will, accept North Korea as a nuclear state. Far from achieving its stated national security and economic development goals, North Korea’s provocative and destabilizing actions have instead served to isolate and impoverish its people through its relentless pursuit of nuclear weapons and ballistic missile capabilities.

For their part, the North Koreans expressed outrage at the “racket of threat and sanctions… kicked up by the U.S.-led hostile forces” to “deny a sovereign state’s exercise of the right to self-defense” in their statement on the nuclear bomb test, as quoted by the BBC.

The BBC’s analysis notes that strong sanctions are already in place against the regime in Pyongyang, and the imposition of anything tougher, such as “blocking the export of fuel oil to North Korea,” would be a “drastic step which might halt the economy and cause serious suffering to ordinary people.”

Such drastic measures would probably be opposed by China, which is haunted by the thought of North Korea collapsing, destabilizing the region, and flooding China with refugees.

Also, while North Korea is noted for issuing a constant stream of apocalyptic threats with clockwork regularity, a sanctions regime that actually shut down their economy probably would be interpreted as an act of war. Pyongyang has also portrayed the deployment of America’s THAAD anti-missile system to South Korea, mentioned by President Obama in his response, as an act of military aggression.

Reuters cited nervousness over the North Korean nuclear test as one reason for Friday’s stock plunge on Friday afternoon, saying the S&P 500 was “on pace for its worst day in over two months.”