North Korea said today it successfully tested a nuclear weapon in what’s believed to be the country’s most powerful nuclear blast to date. In a statement, North Korea claimed today’s underground test proved it had a weapon standardized to fit on a ballistic missile. A news reader with state-run television KRT made the announcement.

KRT news reader: “Our scientists and technicians of the Nuclear Weapons Institute carried out a nuclear explosion test at the northern nuclear test ground to check the power of a nuclear warhead which is newly studied and manufactured. It was confirmed that there was not a leak of radioactive substances during the test, therefore, it did not have negative effects on the ecological environment of the surroundings.”

The U.S. Geological Survey says it monitored an earthquake of magnitude 5.3 at North Korea’s nuclear test site. South Korean officials says the blast appeared to be twice as powerful as a test in January, but much too small to be a hydrogen bomb. Japan’s prime minister called the nuclear test a “grave threat” and promised action at the U.N. Security Council. And South Korea’s military issued a warning to North Korea. This is Leem Ho-young of South Korea’s Joint Chiefs of Staff.

Leem Ho-young: “Our military will not tolerate North Korea conducting a nuclear test again. As we warned it before, we will take all available measures to make North Korea abandon its nuclear program.”

The White House says President Barack Obama spoke with his counterparts in Japan and South Korea about the nuclear test. Spokesperson Josh Earnest said “provocative actions” by North Korea would be met with “serious consequences.”