"It's hard for us to verify their claim. My deep fear is that they will launch a live nuclear weapon on one of their missiles, but that would be extremely dangerous as that could trigger a war," said Melissa Hanham, a senior research associate at the James Martin Center for Nonproliferation Studies
World leaders condemned the test
, saying it was a clear violation of UN Security Council resolutions. The Security Council met and "strongly condemned" the test as "a clear threat to international peace and security."
"The members of the Security Council also recalled that they have previously expressed their determination to take further significant measures in the event of another Democratic People's Republic of Korea nuclear test," the Security Council statement said. "In line with this commitment and the gravity of this violation, the members of the Security Council will begin to work immediately on appropriate measures. ..."
U.S. President Barack Obama warned North Korea that it will face "consequences to its unlawful and dangerous actions."
Obama said he spoke to the leaders of South Korea and Japan by phone, and that they agreed to "take additional significant steps, including new sanctions."
"To be clear, the United States does not, and never will, accept North Korea as a nuclear state," Obama said in a statement.
The blast, detected about 9 a.m. local time (8:30 p.m. ET) in the area of North Korea's Punggye-ri underground test site, is estimated to have had the explosive power of 10 kilotons, almost twice as large as the most recent test in January
, said Kim Nam-wook of South Korea's Meteorological Administration.
The test comes on North Korea's national day, and six months after it claimed it had miniaturized nuclear warheads to fit on ballistic missiles
. At the same time, South Korea is negotiating an expected deployment of a US missile defense system
-- a plan that Pyongyang has decried as provocative.
South Korean President Park Geun-hye called Kim Jong Un's regime "fanatically reckless."
"The only thing that (the) Kim Jong Un regime can gain from the nuclear tests is stronger sanctions from the international community and its isolation. Such provocation will eventually hasten its path to self-destruction," she said in a statement.
Sanctions against the regime, including ones targeting the North Korean leader personally, have had little effect.
Japanese Prime Minister Shinzo Abe told reporters that it was "absolutely unacceptable" if North Korea had conducted another nuclear test.
Obama, who just returned from Asia
, called the test a grave threat to international security. He said he reaffirmed to Park and Abe the "unshakeable US commitment to take necessary steps to defend our allies in the region," including through the planned deployment of the Terminal High Altitude Area Defense, or THAAD
, anti-missile system in South Korea.
Same location as other tests
Seismic activity was detected near Punggye-ri,
the underground site
used in the other four tests.
The US Geological Survey reported a 5.3-magnitude earthquake but later termed it an explosion.
"We are currently analyzing whether it was a successful test," a South Korean National Defense Ministry official told CNN.
Satellite images had shown new activity at the test site in recent weeks, according to North Korea monitoring site 38North
. A small number of mining carts could be seen, as well as a new canopy designed to hide activity to the site, analysts said.
How close is North Korea to mounting a nuclear warhead?
North Korea has continued to improve its nuclear and missile capabilities, but it has yet to pair the two successfully. However, concern has been growing
that the country is testing weapons at an unprecedented pace this year, CNN international correspondent Paula Hancocks said.
South Korea worries that Pyongyang is progressing toward its goal of mounting a nuclear warhead to ballistic missiles
"faster than previously estimated," a South Korean lawmaker told reporters.
Kim Byung-kee of the opposition Minjoo Party made the remark after attending a meeting with South Korean intelligence officials.
Reporters asked him whether the test means North Korea can now mount a nuclear warhead.
"It doesn't appear to be the case," Kim said. "Even if they can mount it (on a missile), the technology for weaponizing it is a separate issue. It doesn't appear they will be able to complete (this) in the next one or two years, but the intelligence authorities said it may be able to be weaponized sooner than expected."
Earlier tests and sanctions
In January, North Korea claimed it had successfully tested a hydrogen bomb, a move the Security Council condemned and that led to punitive sanctions on the North Korean regime. But a U.S. official directly familiar with an assessment of the test said in February there may have been a partial, failed test of some type of components associated with a hydrogen bomb.
A hydrogen bomb, also known as a thermonuclear bomb, can be hundreds of times more powerful than an atomic bomb, like the one dropped on Hiroshima during World War II. While the Hiroshima explosion produced the equivalent of about 15,000 tons of TNT, the world's first thermonuclear test, conducted by the United States in the Marshall Islands in 1952, yielded the equivalent of 10.4 million tons of TNT, a blast 700 times more powerful.
In addition to January's nuclear test, North Korea has tested a number of ballistic missiles, including some launched from a submarine
last month and three more launched from land this week.
Previous Security Council resolutions prohibit North Korea from conducting nuclear tests and launching ballistic missiles. After North Korea's test in January, the council imposed a round of sanctions two months later.
The sanctions included banning Pyongyang from exporting most of its natural resources, prohibiting the supply of aviation fuel and the sale of small arms to North Korea, and requiring the inspection of all North Korean planes and ships carrying cargo abroad.