US Defence Secretary Jim Mattis has reacted to North Korea's latest nuclear test by saying threats to the United States and its allies "will be met with a massive military response".
Mr Mattis spoke at the White House following a meeting with President Donald Trump and national security advisers. He said any response will be "both effective and overwhelming".
Mr Mattis said the United States is "not looking to the total annihilation" of North Korea, but added "we have many options to do so".
North Korea claimed "perfect success" in an underground test of what it called a hydrogen bomb - potentially vastly more destructive than an atomic bomb.
It was the North's sixth nuclear test since 2006, but the first since Mr Trump took office in January.
The UN has joined condemnation of North Korea's latest nuclear weapons test - saying it is "profoundly destabilising for regional security."
The French and German leaders have issued a joint statement condemning the test and British Prime Minister Theresa May says it is an 'unacceptable further threat'.
Meanwhile - Donald Trump has replied "We'll see" - when asked by reporters if the US would attack North Korea.
Michael Fuchs is President Obama's former advisor on the country - he is critical of the Mr Trump's handling of the situation so far.
"One of his earlier tweets from this morning was to criticise North Korea for its quote 'talk of appeasement'. That is exactly the opposite of the kind of reassurance that our South Korean allies need right now," said Mr Fuchs.
Donald Trump has replied "We'll see" - when asked if the US would attack North Korea after its latest nuclear weapons test.
America says it is preparing a new sanctions package, potentially to cut off all trade with the secretive state, following the explosion overnight.
The British Prime Minister Theresa May, and Foreign Secretary Boris Johnson, have condemned the test, calling it "reckless."
The French and German leaders have issued a joint statement condemning North Korea's latest nuclear test - and urging the UN security council to respond.
Pyongyang has claimed to have carried out the 'perfect' detonation of a hydrogen bomb with geologists suggesting the resulting blast was 10 times bigger than any of the country's five previous tests.
North Korea's latest nuclear test poses an "unacceptable further threat to the international community", Theresa May said, as she urged world leaders to increase pressure on Pyongyang.
The Prime Minister said the test was "reckless" and said the case for tougher action against Kim Jong Un's regime was now even more pressing.
Her comments came after Foreign Secretary Boris Johnson warned that North Korea could present a "new order of threat" if it succeeds in producing a hydrogen bomb small enough to fit on a ballistic missile.
Theresa May said North Korea was guilty of "reckless provocation" over its latest missile launch pic.twitter.com/2nNHEVz5Ul— The Telegraph (@Telegraph) August 29, 2017
The test came after propaganda pictures were published of Kim examining what was said to be a nuclear warhead being fitted on to the nose of an intercontinental ballistic missile.
The Prime Minister said: "This latest action by North Korea is reckless and poses an unacceptable further threat to the international community."
Mrs May said she had discussed the "serious and grave threat these dangerous and illegal actions present" with Japanese Prime Minister Shinzo Abe during her visit to the country last week and reiterated their joint call for "tougher action, including increasing the pace of implementation of existing sanctions and looking urgently in the UN Security Council at new measures".
She said: "This is now even more pressing. The international community has universally condemned this test and must come together to continue to increase the pressure on North Korea's leaders to stop their destabilising actions."
US President Donald Trump has branded North Korea "a rogue nation" whose "words and actions continue to be very hostile and dangerous" to the United States.
It comes after North Korea detonated a nuclear device in its sixth and most powerful test to date.
Mr Trump tweeted that North Korea "has become a great threat and embarrassment to China, which is trying to help but with little success".
Though the precise strength of the blast has yet to be determined, the artificial earthquake it caused was several times stronger than tremors generated by its previous tests. It reportedly shook buildings in China and in Russia.
The test was carried out at 12.29pm local time at the Punggye-ri site where North Korea has conducted nearly all of its past nuclear tests. Officials in Seoul put the magnitude at 5.7 while the US Geological Survey said it was a magnitude 6.3.
China's foreign ministry said that the Chinese government has "expressed firm opposition and strong condemnation". It urged North Korea to "stop taking erroneous actions that deteriorate the situation".
South Korea held a National Security Council meeting chaired by President Moon Jae-in. National Security Director Chung Eui-yong said Mr Moon will seek every available measure, including new UN sanctions or the deployment of more US military assets, to further isolate Pyongyang.
Japan's Prime Minister Shinzo Abe called the test "absolutely unacceptable".
French President Emmanuel Macron condemned "in the strongest possible terms" North Korea's sixth nuclear test.
Mr Macron said he "calls on the members of the United Nations Security Council to quickly react to this new violation by North Korea of international law".
He said the international community "must treat this new provocation with the utmost firmness" to bring North Korea back to the path of dialogue and give up its nuclear and missile programmes.
The Russian Foreign Ministry said North Korea's claim to have tested a hydrogen bomb "deserves the strongest condemnation".
A spokesman called for immediate dialogue and negotiations, saying that is the only way to settle the Korean Peninsula's problems, "including the nuclear one".
The ministry said Russia reaffirms its readiness to participate in negotiations, "including in the context of the implementation of the Russian-Chinese road map".
Under that proposal, North Korea would suspend nuclear and missile tests in exchange for the US and South Korea suspending their joint military exercises.
North Korea has successfully conducted a test of a nuclear bomb that is meant to be loaded into an intercontinental ballistic missile.
State TV announced the test's success hours after Seoul and Tokyo detected unusual seismic activity at North Korea's nuclear test site.
The TV anchor said North Korea's leader Kim Jong Un ordered the test.
South Korea's presidential office said the security chiefs for Seoul and Washington have spoken following North Korea's sixth nuclear test.
A spokesman said US National Security Adviser HR McMaster spoke with his South Korean counterpart, Chung Eui-yong, for 20 minutes in an emergency phone call about an hour after the detonation.
South Korea's weather agency estimated the nuclear blast yield of the presumed test was between 50 and 60 kilotons, or five to six times stronger than North Korea's fifth test in September 2016.
That would mark a significant step forward in the North's quest for a viable nuclear missile capable of striking anywhere in the United States.
On North Korean television, a newsreader called the test a "complete success" and said the "two-stage thermonuclear weapon" had "unprecedented" strength.
Hours earlier, Pyongyang claimed its leader had inspected a hydrogen bomb meant for a new intercontinental ballistic missile.
Seoul's weather agency and the Joint Chiefs of Staff said an artificial 5.7 magnitude quake occurred at 12.29pm local time, in Kilju, northern Hamgyong province, the site where North Korea has conducted nuclear tests in the past.
Japan confirmed that North Korea conducted a nuclear test, Foreign Minister Taro Kono said. "It is absolutely unacceptable if North Korea did force another nuclear test, and we must protest strongly," Prime Minister Shinzo Abe said.
North Korea conducted two nuclear tests last year, the last nearly a year ago, on the September 9 anniversary of the nation's founding.
It has since maintained a fast pace in weapons tests, including its first two intercontinental ballistic missiles test in July. Last month, North Korea fired a potentially nuclear-capable mid-range missile over northern Japan.
Earlier on Sunday, photos released by the North Korean government showed Kim talking with his lieutenants as he observed a device that was apparently the thermonuclear weapon destined for an ICBM.
Another photo showed a diagram on the wall behind Kim of a bomb mounted inside a cone.
State media said Kim visited the Nuclear Weapons Institute and inspected a "homemade" H-bomb with "super explosive power" that "is adjustable from tens (of) kiloton to hundreds (of) kiloton".
North Korea's nuclear and missile programme has made huge strides since Kim rose to power following his father's death in late 2011. The North followed its two tests of Hwasong-14 ICBMs by threatening in August to launch a salvo of its Hwasong-12 intermediate range missiles toward the US Pacific island territory of Guam.
It flew a Hwasong-12 over northern Japan last week, the first such overflight by a missile capable of carrying nukes, in a launch Kim described as a "meaningful prelude" to containing Guam, the home of major US military facilities, and more ballistic missile tests targeting the Pacific.
It may be difficult for outside experts to confirm that the nuclear device detonated Sunday was an H-bomb. State media reported that the test left no trace of radioactive material.
The US and its allies attempt to detect blast material to gauge North Korea's progress, but Pyongyang has become better at containing it as its nuclear programme has evolved.