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 told CNN it could take days to obtain the scientific data to determine whether this was a successful test.

The South Korean defense ministry said it too could not immediately confirm the test's success, but the country's foreign ministry hastily convened an emergency meeting. Officials in Japan were also holding discussions.

The test took place at 10 a.m. local time, the regime said in a televised statement.

The seismic event, which measured the event at a magnitude of 5.1, occurred 19 kilometers (12 miles) east-northeast of Sungjibaegam, the United States Geological Survey said.

JUST WATCHED What weapons does North Korea have? Replay More Videos ... MUST WATCH What weapons does North Korea have? 03:43

A big 'if'

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 typically have a yield of around 10 kilotons, while nuclear weapons employing fusion can have a yield measured in megatons.

A hydrogen bomb is hundreds of times more powerful than the atomic bomb that devastated Hiroshima in 1945.

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 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."

JUST WATCHED Who is Kim Jong Un? Replay More Videos ... MUST WATCH Who is Kim Jong Un? 01:57

Regional response

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."

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.

"The fact that the test has taken place, assuming it was successful, complicates the situation in Northeast Asia," he said. "Beijing had been becoming more friendly."

Being more warm and cordial was hoped to restrain North Korea but now this places the Chinese authorities in a big dilemma.

South Korea has also said a fourth test would be a watershed moment that would warrant a response, Chinoy said.

There is currently no diplomacy from the U.S. to restrain the nuclear development, so this test "also puts the U.S. on the spot.

"Will any of their steps do anything to restrain North Korea? My guess is probably not."

Japan quickly issued a strong condemnation, saying the test was a "serious threat" to its security.

"It clearly violates the UNSC resolution and is a serious challenge to the nuclear non-proliferation efforts," said Japanese Prime Minister Shinzo Abe.

A UN diplomat tells CNN there is expected to be closed-door session Security Council consultations Wednesday morning (ET).

Heavily militarized country

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.