North Korea on Wednesday announced that it has successfully conducted a hydrogen bomb test, hours after what seems to be an artificial earthquake was detected close to the nuclear test site.
In a "special" announcement, North Korea said it conducted an H-bomb test, Yonhap reported.
The test means a higher stage of North Korea's development of nuclear force, Pyongyang said.
By succeeding in the H-bomb test, North Korea "joined the advanced ranks of nuclear weapons states possessed of even H-bomb", it said.
Seoul denounced Pyongyang's hydrogen bomb test as a violation of the UN Security Council resolutions.
Vice Foreign Minister Lim Sung-nam said: "North Korea's provocation is in clear violation of Security Council resolutions and a serious challenge to international peace and security."
Seoul has put its military on alert and beefed up surveillance of North Korea after the recent announcement by the communist country.
"Regarding the situation, South Korea and the US are closely cooperating," the defence ministry said, adding that it has also launched an emergency crisis management body.
President Park Geun-hye will preside over a National Security Council meeting in Seoul.
Meanwhile, the White House said it cannot confirm North Korea's claims of a successful H-bomb test, but condemns any violation of the UN resolutions banning Pyongyang from nuclear tests.
We have consistently made clear that we will not accept it as a nuclear state. We will continue to protect our allies in the region, including South Korea, and will respond appropriately to any and all the provocations of North Korea, it said in a statement.
North Korea's announcement came as a 5.1-magnitude earthquake was detected in an area 49 km north of Kilju, home to North Korea's nuclear test site.
Last month, North Korean leader Kim Jong-un said Pyongyang has become a "powerful nuclear weapons state ready to detonate a self-reliant A-bomb and H-bomb".
It was the first time that Kim publicly claimed the development of a hydrogen bomb, which is much more powerful than the conventional nuclear weapons.
South Korea and North Korea ended their rare high-level talks on December 12 without any agreement over how to mend their strained relations.
