The United Nations has strongly condemned North Korea's latest nuclear test and promised new measures against Pyongyang.
North Korea said it had conducted a "higher level" nuclear test explosion that will allow it to build an array of stronger, smaller and lighter nuclear weapons.
The North's fifth atomic test and the second in eight months brought the UN's most powerful body into emergency session, three days after it strongly condemned Pyongyang's latest ballistic missile launches.
UN secretary-general Ban Ki-moon said reducing tensions on the Korean peninsula is one of the most serious issues facing the world because the impact of North Korea's use of nuclear weapons would be far greater than the casualties in Syria and other conflicts.
He said that for almost 10 years as UN chief, and before that as South Korea's foreign minister, he had tried with "all my efforts ... to talk with North Koreans in any way I can to promote peace and security and reconciliation between the South and the North".
"But I regret to tell you that it has not been materialised because of many different situations, mainly caused by North Koreans' provocative actions," Mr Ban said.
Now, he said, "we are coming to almost this confrontational situation".
South Korea's president said the detonation, which Seoul estimated was the North's biggest in explosive yield, was an act of "fanatic recklessness" and a sign that leader Kim Jong Un "is spiralling out of control".
President Barack Obama condemned the test and said the US would never accept the country as a nuclear power.
North Korea's boast of a technologically game-changing test defied tough international sanctions and long-standing diplomatic pressure to curb its nuclear ambitions.
It will raise serious worries in many world capitals that North Korea has moved another step closer to its goal of a nuclear-armed missile that could one day strike the US mainland.
A press statement agreed on by all 15 UN Security Council members said diplomats will draft a new resolution in response to its earlier promise to take "further significant measures" if the North continues to defy the international community.
"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" in a new UN resolution, the statement said.
The measures will be under Article 41 of the UN Charter, which specifies non-military actions including sanctions.
US ambassador Samantha Power said the council must use "every tool at its disposal" including new sanctions "to demonstrate to North Korea that there are consequences for its unlawful and dangerous actions".
"This is more than brazen defiance," Ms Power told reporters at the UN headquarters. "North Korea is seeking to perfect its nuclear weapons and their delivery vehicles so they can hold the region and the world hostage under threat of nuclear strikes."
What measures are included in a new resolution will largely depend on China, the North's major ally and neighbour which fears any instability on the Korean peninsula.
"All sides should refrain from mutual provocations and any actions that might be a threat to peace and security," China's UN ambassador Liu Jieyi said after the meeting.
"We believe it is more urgent than ever to work together to achieve de-nuclearisation of the Korean peninsula (and) "to prevent proliferation and ... maintain peace and stability on the Korean peninsula."
In March, the Security Council adopted its toughest sanctions against North Korea in two decades in response to a nuclear test in January and a rocket launch. It took two months of negotiations mainly between the US and China.
South Korea's UN ambassador Oh Joon said he hopes agreement on a new resolution will come quickly.
Hours after South Korea noted unusual seismic activity near North Korea's north-eastern nuclear test site, the North said in its state-run media that a test had "finally examined and confirmed the structure and specific features of movement of (a) nuclear warhead that has been standardised to be able to be mounted on strategic ballistic rockets".
"The standardisation of the nuclear warhead will enable (North Korea) to produce at will and as many as it wants a variety of smaller, lighter and diversified nuclear warheads of higher strike power. This has definitely put on a higher level (the North's) technology of mounting nuclear warheads on ballistic rockets."