A large earth tremor has been detected in North Korea, raising speculation that the country has carried out its sixth nuclear test, reports BBC.
US seismologists said the 6.3 magnitude quake in the north-east of the country was a "possible explosion".
South Korea immediately convened a national security council meeting.
The tremor comes hours after North Korean leader Kim Jong-un was pictured with what state media said was a new type of hydrogen bomb.
State media said the device could be loaded on to a ballistic missile. Neither claim could be independently verified.
South Korea officials said the quake took place in Kilju County, where the North's Punggye-ri nuclear test site is situated.
China's Earthquake Administration described the tremor as a "suspected explosion".
Initial reports from the US Geological Survey put the tremor at 5.6 magnitude with a depth of 10km (six miles) but this was later changed to 6.3 magnitude at 0km.
North Korea carried out its last nuclear test in September last year. It has defied UN sanctions and international pressure to develop nuclear weapons and to test missiles which could potentially reach the mainland US.
A series of recent missile tests has caused growing international unease.
In a report on Sunday, the North's state news agency KCNA said Kim Jong-un had visited scientists at the nuclear weapons institute and "guided the work for nuclear weaponisation".
"The institute recently succeeded in making a more developed nuke," the report said, adding: "He (Kim Jong-un) watched an H-bomb to be loaded into a new ICBM (intercontinental ballistic missile)."
The report carried pictures of the leader inspecting the device. It described the weapon as "a multi-functional thermonuclear nuke with great destructive power which can be detonated even at high altitudes".
International experts say the North has made advances in its nuclear weapons capabilities but it is unclear if it has successfully miniaturised a nuclear weapon it can load on to a missile.
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