North Korea's leader is notoriously unpredictable, correspondents say
The UN Security Council has agreed to begin work immediately on a new resolution on North Korea after its nuclear test, its chairman said.
Members voiced strong opposition to the test and condemned it, Russia's UN envoy, Vitaly Churkin, said after an emergency meeting of the Council.
North Korea announced the underground test as part of work to bolster what it called its nuclear deterrent.
The US ambassador to the UN said America wanted "strong measures".
Susan Rice said she expected the Security Council to start working on the resolution on Tuesday.
Churkin said the Security Council had agreed to begin work on a new resolution
"The US thinks that this is a grave violation of international law and a threat to regional and international peace and security," she said, speaking after Mr Churkin in New York.
"And therefore the United States will seek a strong resolution with strong measures."
More missile tests
North Korea had been warning for weeks that it would strengthen its "deterrent" and walked away from long-running negotiations on its nuclear programme after the US pressed it over the verification of previous agreements.
The blast, which was estimated by international seismologists to have the power of a 4.5 earthquake, appears to have been much more powerful than North Korea's first nuclear test, in October 2006.
Defence officials in neighbouring Russia say it was an explosion of up to 20 kilotons, making it comparable to the American bombs that flattened Hiroshima and Nagasaki in 1945.
Meanwhile, a South Korean report warned that the North was ready to fire another short-range missile off its west coast.
"North Korea declared a ban for ships in the sea off its South Pyongan province between the 25th to 27th [of May]. It looks like it will fire a short-range missile between today and tomorrow," Yonhap news agency quoted an unnamed South Korean official as saying.
Seoul has said it will join the Proliferation Security Initiative, a US-led body launched in 2003 which aims to prevent the spread of nuclear and chemical weapons.
Hitherto, South Korea has only been an observer.
Russia's UN envoy told reporters the nuclear test was a clear violation of UN Resolution 1718. That resolution imposed sanctions on North Korea after its first test.
Oct 2006 - North Korea conducts an underground nuclear test
Feb 2007 - North Korea agrees to close its main nuclear reactor in exchange for fuel aid
June 2007 - North Korea shuts its main Yongbyon reactor
June 2008 - North Korea makes its long-awaited declaration of nuclear assets
Oct 2008 - The US removes North Korea from its list of countries which sponsor terrorism
Dec 2008 - Pyongyang slows work to dismantle its nuclear programme after a US decision to suspend energy aid
Jan 2009 - The North says it is scrapping all military and political deals with the South, accusing it of "hostile intent"
April 2009 - Pyongyang launches a rocket carrying what it says is a communications satellite
25 May 2009 - North Korea conducts a second nuclear test
"The members of the Security Council voiced their strong opposition to, and condemnation of, the nuclear test conducted by the Democratic People's Republic of Korea on May 25 2009, which constitutes a clear violation of resolution 1718," Mr Churkin said.
"The members of the Security Council demand that the DPRK [North Korea] comply fully with its obligations under resolutions 1695 and 1718, and other related Security Council resolutions and statements, and calls upon all member-states to uphold their obligations under these resolutions and statements.
"The members of the Security Council have decided to start work immediately on a Security Council resolution on this matter, in accordance with the Security Council's responsibilities under the Charter of the United Nations."
The UK's Ambassador to the UN, Sir John Sawers, explained that the Council had decided to act in two stages, first issuing a statement "strongly condemning and opposing what the North Koreans have done by carrying out this second nuclear test".
"And we decided to start work immediately on a further Security Council resolution in order to uphold the international peace and security in the region," he added.
But in a strongly-worded statement on Tuesday, North Korea said it was fully prepared for any "pre-emptive" attack by the United States.
"It is clear that nothing has changed in the US hostile policy against DPRK [North Korea]... even under the new US administration," the North's KCNA news agency said in an article criticising recent US moves to relocate its fighter jets.
"Our army and people are fully ready for battle... against any reckless US attempt for a pre-emptive attack."
The international community faces a difficult task, confronting what many view as a dangerously unpredictable regime, BBC world affairs correspondent David Loyn reports.
After the worst harvest for a decade, the World Food Programme says a quarter of the population of North Korea needs food aid.
To divert attention, the government has abruptly broken off talks, test-firing both long- and short-range missiles and then carrying out the underground test, a major escalation, our correspondent says.
It could be that this change of mind is part of an internal power struggle as hardliners seek to provoke the world to justify their own repressive rule, he adds.
An official communique read out on North Korean state radio said another round of underground nuclear testing had been "successfully conducted... as part of measures to enhance the Republic's self-defensive nuclear deterrent in all directions".
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