Seoul: North Korea announced on Wednesday that it had successfully carried out its first hydrogen bomb test, a development that, if confirmed, would marking a stunning step forward in its nuclear development.
"The republic's first hydrogen bomb test has been successfully performed at 10 am on 6 January, 2016, based on the strategic determination of the Workers' Party," a state television news reader announced.
"With the perfect success of our historic H-bomb, we have joined the rank of advanced nuclear states," the announcer said, adding that the test was of a "miniaturised" device.
The surprise test was personally ordered by North Korean leader Kim Jong-Un and came just two days before his birthday.
Only last month, during remarks made during an inspection tour, Kim had suggested Pyongyang had already developed a hydrogen bomb — although the claim was greeted with scepticism by international experts.
A hydrogen, or thermonuclear device, uses fusion in a chain reaction that results in a far more powerful explosion.
"The latest test, completely based on our technology and our manpower, confirmed that our newly-developed technological resources are accurate and scientifically demonstrated the impact of our miniaturised H-bomb," the TV announcer said.
The announcement will leave the international community scrambling to verify the accuracy of the North's claims.
Most experts had assumed Pyongyang was years from developing a thermonuclear bomb, while assessments were divided on how far it had gone in mastering the technology to miniaturise a device that could fit on a ballistic missile.
While vowing to stick by a no-first use policy, Wednesday's statement said Pyongyang would continue to pursue an advanced nuclear strike capability.
"As long as the vicious anti-North policy of the US persists, we will never stop development of our nuclear programme," it said
Suspicions over a possible nuclear test — Pyongyang's fourth — were first raised by seismologists who said they had detected a 5.1 magnitude tremor next to its main atomic test site in the northeast of the country.
The website of the China Earthquake Network Centre described the seismic activity as a "suspected explosion", while the Japanese government said there was a strong possibility that "this might be a nuclear test".
The US Geological Survey said the epicentre of the quake — detected at 10 am Pyongyang time (0130 GMT) — was in the northeast of the country, some 50 kilometres (30 miles) northwest of Kilju city, placing it right next to the Punggye-ri nuclear test site.
Any confirmation of the test will trigger widespread international condemnation of North Korea, which has already conducted three nuclear tests in 2006, 2009 and 2013 -- all at Punggye-ri.
It would certainly result in a tightening of international sanctions imposed after the North's previous nuclear and ballistic missile tests.
In Seoul, the presidential Blue House called an emergency meeting of the National Security Council, as officials scrambled to confirm the precise nature of the tremor.
Researchers at the US-Korea Institute at Johns Hopkins University said last month that recent satellite images showed North Korea was excavating a new tunnel at Punggye-ri.
"While there are no indications that a nuclear test is imminent, the new tunnel adds to North Korea's ability to conduct additional detonations over the coming years if it chooses to do so," they said at the time.
A nuclear test is as a major slap in the face to the North's chief ally China and extinguish any chance of a resumption of six-country talks on North Korea's nuclear programme that Beijing has been pushing for.
After its last nuclear test in 2013, the North restarted a plutonium reactor that it had shut down at its Yongbyon complex in 2007 under an aid-for-disarmament accord.
The Yongbyon reactor is capable of producing six kilograms (13 pounds) of plutonium a year — enough for one nuclear bomb Pyongyang is currently believed to have enough plutonium for as many as six bombs, after using part of its stock for at least two of its three atomic tests to date.
It is still unclear whether the 2013 test used plutonium or uranium as its fissile material.
A basic uranium bomb is no more potent than a basic plutonium one, but the uranium enrichment path holds various advantages for the North, which has substantial deposits of uranium ore.
Uranium enrichment carries a far smaller footprint than plutonium and can be carried out using centrifuge cascades in relatively small buildings that give off no heat.
With inputs from AFP