North Korea claimed Wednesday it had successfully tested a hydrogen bomb earlier in the day, swiftly drawing international condemnation.

Pyongyang’s state-run KCNA made a noon announcement that was also broadcast in South Korea, stating that the historic test had been carried out at 10 a.m. local time (0100GMT) under the orders of leader Kim Jong-un — who last month warned for the first time that his country was “ready to detonate a self-reliant H-bomb.”

Hydrogen bombs involve both fission and fusion reactions to generate even more power than single-stage atomic weapons.

Seoul’s presidential office described what would be North Korea’s fourth ever nuclear test as “a clear violation” of United Nations Security Council (UNSC) resolutions.

President Park Geun-hye added that South Korea would work with other nations to punish Pyongyang.

Deputy chief of national security Cho Tae-yong added that the South would work with other nations to do everything possible to punish Pyongyang.

North Korea has repeatedly faced sanctions for what its neighbor sees as past provocations, including its last nuclear test in 2013, which prompted months of high tensions on the Korean Peninsula.

Meanwhile in the United States, the White House was cautiously monitoring North Korea’s announcement.

“While we cannot confirm these claims at this time, we condemn any violation of UNSC Resolutions,” said a spokesperson.

In the absence of independent verification, global observers did note what appeared to be an artificial earthquake Wednesday morning in the same region as the North’s Punggye-ri test site.

The U.S. Geological Survey recorded the magnitude of the tremor at 5.1 — although a hydrogen bomb test would have been expected to have had more of an impact.

“It is hard to regard this test as that of a hydrogen bomb,” a South Korean military source was quoted as saying by local news agency Yonhap.

The official pointed out that North Korea’s 6-kiloton detonation was far weaker than the 20 to 50-megaton blasts produced by American and Russian tests.

China, which also reported Wednesday’s seismic activity, revealed via its foreign ministry that it would lodge a protest with North Korea.

Despite their traditional alliance, Beijing has been playing a more active role in trying to contain Pyongyang’s nuclear weapon ambitions in recent years (Anadolu Agency).