The UN Security Council is meeting in emergency closed session over North Korea's latest nuclear test after even its allies joined in criticism.
The UN Security Council has agreed to begin work immediately on a new resolution on North Korea after its nuclear test, its chairman said.
US President Barack Obama described North Korea's programme as a "grave threat" to world peace.
"Members voiced strong opposition and condemned the test," Russia's UN envoy, Vitaly Churkin, said after an emergency meeting of the Council in New York.
China accused the North of disregarding international opinion while Moscow said it had raised tension in the region.
The test was, he added, a clear violation of existing UN resolutions.
North Korea said the underground test had been part of work to bolster what it called its nuclear deterrent.
North Korea has said the underground test was part of work to bolster what it called its nuclear deterrent.
It had been warning for weeks that it would strengthen the "deterrent" and walked away from long-running negotiations on its nuclear programme after the US pressed it over the verification of previous agreements. See map of test site
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.
Call for calm
UN Secretary General Ban Ki-moon said he was deeply disturbed by the underground test, not only in his UN capacity but also as a Korean.
NUCLEAR CRISIS Oct 2006 - North Korea conducts an underground nuclear testFeb 2007 - North Korea agrees to close its main nuclear reactor in exchange for fuel aidJune 2007 - North Korea shuts its main Yongbyon reactorJune 2008 - North Korea makes its long-awaited declaration of nuclear assetsOct 2008 - The US removes North Korea from its list of countries which sponsor terrorismDec 2008 - Pyongyang slows work to dismantle its nuclear programme after a US decision to suspend energy aidJan 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 satellite25 May 2009 - North Korea conducts a second nuclear test Q&A: North Korea nuclear testKorean test rocks regionN Korea a problem for Obama
He said North Korea's actions were a clear violation of UN resolutions and ran counter to ongoing efforts to curb nuclear proliferation.
Mr Ban said he hoped the Security Council would take what he called necessary measures.
EU foreign policy chief Javier Solana said he hoped for a "very clear and tough reaction" from the Security Council.
"We expect that China, Russia and the United States, Japan will continue to put all the pressures so that this regime, which is so unpredictable, becomes a predictable one, in the right direction," he added.
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 believe a quarter of the population of North Korea need 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.
That, perhaps, is why China called for a calm and appropriate response - angry but not so angry for Beijing to back sanctions and encourage the hardliners in North Korea.
A measure condemning North Korea will emerge from the UN Security Council but there are few measures available that would bring them back to negotiations and President Obama, the natural conciliator, faces a major challenge, our correspondent says.
'Stronger than 2006'
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".
The time to be worried has long passed Michael, Israel (currently in the US), BBC News website reader Should the world be worried?
Monitors detected the test at a depth of 10km (six miles), at 0954 (0054 GMT) on Monday.
The North did not report the test site but South Korean officials placed it in the north-eastern region around the town of Kilju, the site of North Korea's first nuclear test.
South Korean Defence Minister Lee Sang-hee said the North had also launched two short-range missiles, one at 1208 (0308 GMT), and one at 1703 (0803 GMT).
Mr Obama said the US would work with its allies around the world to "stand up to" North Korea.
"North Korea is not only deepening its own isolation it's also inviting stronger international pressure," he said.
NORTH KOREA'S SUSPECTED NUCLEAR TEST SITE Nuclear test on 25 May 2009 is thought to have been carried out at same site as October 2006 test
