North Korea said Wednesday it had successfully tested a hydrogen bomb for the first time, South Korea's Yonhap News Agency reported, citing North Korean state media.
A hydrogen bomb or thermonuclear weapon delivers much more explosive power than the type of atomic bombs dropped on Hiroshima and Nagasaki at the end of World War II.
South Korea's government called a crisis meeting in Seoul and Japan's security council was scheduled to meet in response to the test.
Japanese Prime Minister Shinzo Abe "strongly condemns" North Korea, he told reporters. The test "is a threat to our country's security and it is totally intolerable."
North Korea carried out three nuclear weapons tests between 2006 and 2013, followed each time by new and stricter UN Security Council sanctions against Pyongyang.
The first sign of the test Wednesday was a tremor that registered as a small earthquake in the region of Kilju in north-east North Korea, where the secretive state's atomic tests have been carried out.
China's earthquake monitoring centre in Beijing said that the shock was of magnitude 4.9. US measurements put the magnitude at 5.1.