HONG KONG -- North Korea says it has successfully carried out a hydrogen bomb test, which if confirmed, will be a first for the reclusive regime and a significant advancement for its military ambitions.
A hydrogen bomb is more powerful than plutonium weapons, which is what North Korea used in its three previous underground nuclear tests.
"If there's no invasion on our sovereignty we will not use nuclear weapon," the North Korean state news agency said. "This H-bomb test brings us to a higher level of nuclear power."
A senior U.S. administration official told CNN it could take days to obtain the scientific data to determine whether this was a successful test. South Korea said it found it difficult to believe North Korea could test a hydrogen bomb, but hastily convened an emergency meeting.
The United Nations Security Council also will hold a meeting later Wednesday, at the behest of the United States and Japan.
A powerful weapon
The test took place at 10 a.m. local time, the regime said in a televised statement.
The seismic event, which measured a magnitude of 5.1, occurred 19 kilometers (12 miles) east-northeast of Sungjibaegam, the United States Geological Survey said.
A document signed by North Korea's leader, Kim Jong Un, seen in footage aired on North Korean state television, read: "Make the world to look up to our strong nuclear country and labor party by opening the year with (the) exciting noise of the first hydrogen bomb!"
In the past, North Korea has tested fission weapons, which break large atoms like plutonium, into smaller atoms, creating considerable energy.
Fusion weapons, such as hydrogen bombs, use fusion to combine small atoms -- such as hydrogen -- to create much larger amounts of energy.
Nuclear weapons based on fission -- such as the atomic bombs that devastated Hiroshima and Nagasaki in 1945 -- typically have a yield of around 10 - 15 kilotons, while nuclear weapons employing fusion can have a yield measured in megatons.
A big 'if'
The North Koreans have signaled for some time the test was a possibility, said Mike Chinoy, with the U.S.-China Institute at the University of Southern California.
"Kim Jong Un made a public statement a few weeks ago saying that (the country was) developing a hydrogen bomb."
But, said Bruce Bennett, North Korea's claims ought to be taken with a grain of salt. Bennett is a senior defense analyst at the nonprofit, nonpartisan Rand Corp.
"North Korea appears to have had a difficult time mastering even the basics of a fission weapon," he said. "This suggests that unless North Korea has had help from outside experts, it is unlikely that it has really achieved a hydrogen/fusion bomb since its last nuclear test, just short of three years ago."
A continuing challenge
The development illustrates the continuing challenge North Korea poses to its neighbors and the world.
"We have consistently made clear that we will not accept it as a nuclear state," said a spokesman for the National Security Council. "We will continue to protect and defend our allies in the region, including the Republic of Korea, and will respond appropriately to any and all North Korean provocations."
Jasper Kim with the Center for Conflict Management at Ewha Womans University in Seoul said the test is an attempt to draw attention to North Korea on the international stage.
"What Kim Jong Un wants is a conversation with the U.S. President," he said. "That's why the test is happening now. That's why the stakes are so high."
Three of the four of North Korea's nuclear tests -- in 2009, 2013 and now -- have taken place while Barack Obama has been the U.S. President.
While the Obama administration has had success in curtailing Iran's nuclear ambitions to a degree, it has been unable to make any headway with North Korea.
This test "puts the U.S. on the spot," Chinoy said.
"Will any of their steps do anything to restrain North Korea? My guess is probably not."
A sticking point
For years, the world has been trying to bring North Korea to the table to talk disarmament, but with little success. The six-party talks are comprised of United States, North Korea, South Korea, China, Japan and Russia.
In lieu of progress have come sanctions, not just for North Korea's nuclear tests but also as a reaction to its alleged human rights abuses.
Recent U.N. resolutions have included arms and non-proliferation embargoes to luxury good embargo, a freeze on overseas financial assets and a travel ban.
But those seem to have had little effect on Pyongyang's ambitions.
"If a nuclear device has been detonated... it underlines the very real threat that North Korea represents to regional and international security," British Foreign Secretary Philip Hammond said.
A spokesperson from China's Ministry of Foreign Affairs said: "China strongly opposes North Korea's H-bomb test. We of course will summon high -level officials and the ambassador (for serious discussion).
South Korea has also said a fourth test would be a watershed moment that would warrant a response.
"This is clearly a provocation and threatening the lives of people and safety," South Korean President Park Geun-hye said.
"We have been continuously warning that (North Korea) will pay a price for conducting a nuclear test."
Heavily militarized country
Gauging the progress of North Korea's nuclear weapons program is a tricky business. Kim's regime generally cloaks its efforts in secrecy and occasionally trumpets claims of advances through propaganda outlets, leaving the rest of the world to try to connect the dots.
North Korea's internationally isolated regime is a heavily militarized state with a huge standing army of 1.2 million active soldiers and 7.7 million reservists.
But its conventional weaponry is dated, with limited effectiveness, and it has looked to developing its nuclear capabilities to project power internationally.
The country declared it had nuclear weapons in 2003, and conducted nuclear tests in 2006, 2009 and 2013.
In May last year, it said it had the ability to miniaturize nuclear weapons, a development that would allow it to deploy nuclear weapons on missiles. A U.S. National Security Council spokesman responded at the time that the United States did not think the North Koreans had such a capability.
David Albright, a former U.N. weapons inspector, told CNN last year that Pyongyang could already have 10 to 15 atomic weapons, and that it could grow that amount by several weapons per year.
He said he believed Pyongyang had the capability to miniaturize a warhead for shorter missiles, but not yet for intercontinental ballistic missiles capable of reaching the United States.