The UN Security Council has strongly condemned North Korea's claim to have tested a nuclear weapon.
The claim of a test drew sharp rebukes from around the world
The council is now considering its next steps, including sanctions that could be enforceable by military means.
The US has proposed a draft resolution which includes powers to inspect cargo and close air and sea ports to North Korean ships and planes.
President George W Bush said the US was working to confirm the test claim, branding it a "provocative" act.
He said he and regional leaders agreed North Korea's actions were unacceptable and deserved an immediate UN response.
Current Security Council President Kenzo Oshima, of Japan, urged North Korea to refrain from further testing and return to six-party talks.
The Americans have circulated a 13-point draft resolution seeking targeted sanctions. The proposals include:
- Halting trade in material that could be used to make weapons of mass destruction
- Inspections of cargo going in and out of North Korea
- The ending of financial transactions used to support nuclear proliferation
- A ban on the import of luxury goods
The US also wants to see the sanctions brought under Chapter Seven of the UN Charter, which means they would be mandatory and ultimately enforceable by military means.
UK Foreign Secretary Margaret Beckett also said London would be "pushing for a robust response" under Chapter Seven.
However, the Russians and Chinese - who have trade links with North Korea - have been reluctant to go down that path, says the BBC's Laura Trevelyan at the UN in New York.
US ambassador to the UN John Bolton told the BBC: "If the Security Council of the United Nations can't deal with a threat like that then we have to ask what role it could have in dealing with weapons of mass destruction around the world.
But North Korea's ambassador, Pak Gil Yon, said the Security Council should congratulate Pyongyang instead of issuing "useless" resolutions.
In his first public statement since the reported test, US President Bush said the North Korean claim "constitutes a threat to international peace and security."
Mr Bush said he had telephoned Chinese, Japanese, Russian and South Korean leaders, who had all reaffirmed their commitment to a nuclear-free Korean peninsula.
"Once again, North Korea has defied the will of the international community, and the international community will respond," he said.
"The North Korea regime remains one of the world's leading proliferators of missile technology, including transfers to Iran and Syria."
Mr Bush added that the development would not help North Korea's "oppressed and impoverished" people, who deserved a better future.
Earlier Japan's Prime Minister Shinzo Abe - during a visit to Seoul - called the claimed test "unpardonable".
He warned the region was "entering a new, dangerous nuclear age".
South Korean President Roh Moo-hyun feared the move could "spark a nuclear arms build-up in other countries".
In an unusually strong statement, China - the closest the isolated North has to an ally - said the claimed test "defied the universal opposition of international society".
Meanwhile, the head of the South's intelligence service said it had detected more movement at another North Korean test site and he could not rule out further nuclear tests.
'No radiation leak'
South Korean media said the test took place in Gilju in Hamgyong province at 1036 (0136 GMT).
The size of the bomb is uncertain, with estimates varying from 550 tons of destructive power to as much as 15 kilotons. The 1945 Hiroshima bomb was 12.5-15 kilotons.
Correspondents say the claimed test does not necessarily mean North Korea has a fully-fledged nuclear bomb, or a warhead that it can deliver to a target.
North Korea's KCNA news agency described the test as an "historic event that brought happiness to our military and people".
It said the test as a success and was "a great leap forward in the building of a great prosperous, powerful socialist nation".
Pyongyang pulled out of the nuclear Non-Proliferation Treaty in 2003 and has refused for a year to attend talks aimed at ending its nuclear ambitions.