North Korea has announced it has conducted a hydrogen bomb test, after an earthquake was detected close to a site previously used for nuclear tests.
State media announced the test after monitors detected a 5.1 magnitude quake close to the Punggye-ri site.
The North is thought to have conducted three previous underground nuclear tests there since 2006.
A hydrogen bomb uses fusion to create a blast far more powerful than that of a more basic atomic bomb.
North Korea said it was "defending itself" against the United States.
Last month, North Korean leader Kim Jong-Un suggested Pyongyang had already developed a hydrogen bomb, although the claim was greeted with scepticism by international experts.
Earlier today, China, Japan and South Korea all said there are indications the tremor was man-made, meaning the North may have carried out a new nuclear test.
South Korean ministers are holding an emergency meeting, the Yonhap state news agency said.
North Korea is believed to have conducted three underground nuclear tests since 2006, all at a site called Punggye-ri.
The new tremor was detected at 12.30pm (NZT) about 50km from Punggye-ri at a depth of about 10km, according to the US Geological Survey.
The USGS said the quake had a magnitude of 5.1. South Korea's meteorological agency measured it at 4.2.
Japan's chief cabinet secretary Yoshihide Suga said that "considering past cases, there is the possibility that this might be a nuclear test by North Korea".