North Korea's nuclear test is "deeply troubling" and "profoundly destabilising for regional security", United Nations Secretary-General said.
Ban Ki-moon said: "This test once again violates numerous Security Council resolutions despite the united call by the international community to cease such activities.
He added it is a "grave contravention of the international norm against nuclear testing."
He demanded that North Korea cease any further nuclear activities and "meet its obligations for verifiable denuclearisation".
It will be possible to coordinate real international action if the Chinese have decided that the North Korean detonation is a "provocation too far", a former British ambassador to North Korea has said.
John Everard, 59, was the British ambassador to the hermit kingdom between 2006 and 2008 - during the period of North Korea's first nuclear test.
China - North Korea's most powerful ally - has said that it will work with the international community to denuclearise the Korean peninsula, Mr Everard said.
Mr Everard said the Chinese will have to show "at least part of their hand" at the United Nations Security Council meeting tonight.
If the Chinese have decided that this is a provocation too far and from here on the North Koreans have to be brought to heel, then it is possible to coordinate real meaningful international action, but we don't yet know if the Chinese are at that point.
However, he said the Chinese are worried that destabilising the Korean Peninsula could add to economic and social problems in its own regions.
Why is this latest development in North Korea's quest for nuclear power seen as potentially significant?Read the full story ›
Japanese monitoring posts haven't detected any radiation from North Korea's nuclear test, Japan's top government spokesman Yoshihide Suga has said.
South Korea's military have played down the apparent successful test of a North Korean nuclear weapon, saying that that tremor it caused wasn't strong enough for a H-bomb.
A source, who asked to remain anonymous, told the Yonhap state news agency in South Korea that 5.1 magnitude earthquake apparently caused by the bomb was too slight to be a hydrogen device:
"It is hard to regard this test as that of a hydrogen bomb.
Only a few countries including the US and Russia have conducted hydrogen bomb tests and size of detonations reached 20 to 50 megatons."
The latest North Korean test amounted to around six kilotons and is "too weak" for a h-bomb, the official claimed.
But announcing the test, Pyongyang said it had successfully detonated a "miniaturised" device, which could explain its relatively small size in comparison to conventional nuclear weapons.
North Korea would possibly consider a nuclear strike if they thought they could survive the inevitable counter-strike, a former British ambassador to the country has said.
Speaking on BBC Radio 4's Today programme, John Everard said:
If they believe they can survive a nuclear counter-strike, the possibility that they may launch a nuclear first strike remains.
Pyongyang's claims that it has detonated a miniaturised hydrogen-bomb have been condemned resoundingly by the international community.
Some analysts suggest the device may have been between 6 and 12 kilotons - smaller in size than the atomic bomb dropped on Hiroshima by the US in 1945 (16kt).
China had no advanced knowledge of North Korea's reported test of a miniaturised hydrogen bomb, the foreign ministry in Beijing has said.
Speaking during a daily news conference, Ministry spokeswoman Hua Chunying added that China firmly opposes the actions of North Korea.
Ms Chunying also said China would lodge a protest with the government in Pyongyang over the test.
Weather patterns mean that any radioactive material released from the apparent North Korean hydrogen-bomb test is unlikely to spread over South Korea, the country's weather service has said.
The Yonhap news agency, quoting the South Korean meteorological administration, said that if any "radioactive substances" were released during the test, prevailing winds will push them eastwards, over the Sea of Japan.
France has strongly condemned North Korea's apparent hydrogen bomb test, calling for a "strong reaction from the international community" in reaction, President Francois Hollande's office said in a statement.
The statement called the reported test "an unacceptable violation of UN Security Council resolutions".
The reported test potentially marks an advance in North Korea's ability to create a "first strike" nuclear weapon, which has prompted alarm across the world, notably in South Korea and Japan.
South Korea will take decisive measures against additional provocations by North Korea, the country's president Park Guen-hye has said.
In published remarks from his presidential office, Mr Park also said that his country would work with the international community to ensure North Korea pays the price for its latest claims.
The country's vice minister later expressed scepticism over the claim, saying it was difficult to believe the weapon tested was a hydrogen bomb.