Nations around the world on Wednesday condemned North Korea's announcement that it had successfully tested a hydrogen bomb, while Japan said no radiation had been detected at Japanese monitoring posts.
China, North Korea's strongest ally, said it would "launch solemn representations" with Pyongyang, following a claim by the rogue state that it had successfully conducted a test of a miniaturized hydrogen nuclear device — a claim that has been met with skepticism.
If confirmed, the test would amount to a gross violation of international law, said the European Union and Russia's Foreign Ministry on Wednesday. Britain, France, and Japan condemned the test.
The South Korean Presidential Office's senior security official said his government would take all possible measures, including possible UN sanctions, to ensure the North pays the price after the fourth nuclear test. Seoul's defense ministry said it had put its military on alert.
Japan also said it could propose various economic and other sanctions against North Korea to the UN in cooperation with the United States, but did not give details.
However South Korea's Defense Ministry said it was difficult to believe that a fully-fledged hydrogen bomb (H-bomb) had been tested, and nuclear weapons experts told VICE News it was probable that this test was of a device similar to previous weapons tested by North Korea.
Japan's chief cabinet secretary Yoshihide Suga told a news conference on Wednesday that, "As of 4.45 this afternoon, no radiation has been detected at any Japanese monitoring posts."
While it is possible the South Asian state has the materials required to make a hydrogen bomb, it is unlikely they have the technology to make it successfully detonate, said Dr. Ferenc Dalnoki-Veress, a nuclear physicist who is a scientist in residence at the Middlebury Institute of International Studies at Monterey.
H-bomb explosions are several orders of magnitude larger than the type of nuclear weapons used by the US on Hiroshima and Nagasaki during World War II. Those devices, which are relatively crude by modern standards, produced explosions that were the equivalent of less than 20 kilotons of TNT. Today's H-bombs are capable of producing blasts greater than 50,000 kilotons.
Related: Kim Jong-un Says North Korea Has an H-Bomb — But No One Believes Him