North Korea said Wednesday it conducted a "successful" hydrogen bomb test.
The announcement on state television came less than a month after North Korean leader Kim Jon Un claimed his country had developed a hydrogen bomb. The United States and others quickly discounted that claim.
A hydrogen bomb, also known as a thermonuclear bomb, is far more powerful than a atomic bomb.
North Korea conducted three previous nuclear tests in 2006, 2009 and 2013. All of them took place at the Punggye-ri site, the same area where several monitoring agencies detected man-made earthquake activity on Wednesday morning.
The test prompted emergency national security meetings in South Korea and Japan.
Japanese Prime Minister Shinzo Abe condemned North Korea's test, calling it a serious threat to Japan's national security.