North Korea has reportedly tested a miniaturised hydrogen bomb, a move which will heighten tensions between the politically-isolated country and its neighbours, particularly Japan and South Korea.
The nuclear test, its fourth since 2006, came to light after reports of a 5.1 magnitude earthquake near a nuclear testing site in the east Asian country.
"The first H-bomb test was successfully conducted at 10.00am (1.30am GMT) on Wednesday," North Korea's official KCNA news agency subsequently said.
Miniaturising, a process enabling the device to be adapted as a weapon, would pose a new threat to the US and its regional allies, Japan and South Korea.
North Korea said that it would not give up its nuclear capability unless the US drops its hostile foreign policy towards the country.
China, North Korea's main ally, has not commented on the test but is likely to be displeased at the increase in tensions in its neighbourhood
It comes a month after leader Kim Jong Un appeared to claim North Korea had developed a hydrogen bomb, a step up from the less powerful atomic bomb. However, the United States and outside experts voiced sceptism. Similarly, some analysts questioned whether today's test really involved a hydrogen device.
North Korea has been subject to UN Security Council sanctions since it first tested an atomic device in 2006 and could face additional measures.
The Security Council will meet later today to discuss what steps it could take, diplomats said.