North Korea says it has successfully detonated its first hydrogen bomb. If confirmed, the test marks a major increase in North Korea’s nuclear capacity. The North Korean state-run TV network announced the test after seismologists detected a 5.1-magnitude tremor near the country’s main nuclear test site.

Announcer: “The first H-bomb test was successfully conducted at 10:00 on January 6, 2016. We will not give up a nuclear program as long as the United States maintains its stance of aggression.”

North Korea’s announcement has not been independently verified, and experts have voiced skepticism about whether the country is actually capable of producing a hydrogen bomb. South Korea called the announcement a “grave provocation to our national security,” while Japan condemned it as a “major threat to regional and international peace and stability.” Speaking in Tokyo today, U.S. Ambassador to Japan Caroline Kennedy said the United States stands with Japan.

Caroline Kennedy: “We stand with Japan and our partners and allies in solidarity in the face of North Korean provocations, and we will work closely together with you in the coming days.”

Hydrogen bombs can be hundreds or even thousands of times more powerful than the atomic bomb the United States dropped on the Japanese cities of Hiroshima and Nagasaki in 1945—the first and only use of nuclear weapons in war.