The UN Security Council is debating action against North Korea over claims it carried out nuclear tests, with China and Russia urging sanctions.
China, North Korea’s traditional ally, has called for punitive action against the country but - like Russia - it is against the use of military force.
Japan has also urged sanctions, while South Korea said its military was remaining on high alert.
The security council is discussing a US proposal to impose strict sanctions.
The US ambassador to the UN said while the US would not rule out using force, it was seeking a diplomatic solution.
A South Korean seismologist has warned that North Korea could repeat a nuclear test in order to ensure the accuracy of its results.
And a Japanese TV channel reported that Tokyo officials were investigating reports of a further nuclear test following earth tremors in the area.
But South Korean experts said they had detected no seismic activity.
The US has proposed a 13-point draft resolution and 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.
BBC correspondent Laura Trevelyan, who is at the UN headquarters in New York, said there appeared to be some disagreement on the scope of the measures.
The diplomats say they want a resolution quickly, but at this stage it appears one will be reached closer to the end of the week rather than in the next 24 hours.
‘Colossal blow’
Earlier, the South Korean Prime Minister, Han Myung-sook, said Seoul would not support a resolution including a threat of military force.
Russia has described the reported nuclear test as a “colossal blow” but stressed the solution was not military force.
Russia, which like China has resisted sanctions in the past, has said it is “ready to take part in joint efforts of the interested parties to arrive at a peaceful, diplomatic settlement of the situation”.
The response of China - the country that holds the most influence over the isolated regime - is seen by many analysts to be key in moving the crisis forward.
China’s UN ambassador, Wang Guangya, has said North Korea must face “some punitive actions” for conducting a nuclear test.
Chinese foreign ministry spokesman Liu Jianchao said: “This will no doubt have a negative impact on China and North Korea’s relations.”
North Korea’s neighbours remain tense in the wake of Monday’s announcement.
China has reportedly cancelled leave for troops along part of its border with the North, and South Korean forces have been ordered to stay alert.
South Korean President Roh Moo-hyun feared the move could “spark a nuclear arms build-up in other countries” but Japan, the only nation to suffer atomic attack, has pledged that it will not develop nuclear weapons in response.
New Prime Minister Shinzo Abe told the Japanese parliament: “Possession of nuclear arms is not an option at all for our country.”
India, which carried out nuclear tests in 1998, condemned the test, and British Foreign Secretary Margaret Beckett said it “posed a clear threat to international peace and security”.
Only Iran, which also faces Security Council action over its failure to suspend its uranium enrichment programme - has voiced support for North Korea.
The underground test reportedly took place in Gilju in Hamgyong province at 1036 (0136 GMT) on Monday morning.
Confirmation of the bomb has not been made, and a White House spokesman said there was a “remote possibility that we’ll never know” the size of it.
The lack of information over the blast has cast some doubts over North Korea’s claim, with suggestions that the blast may have been produced by conventional explosives as part of a ruse.
But if confirmed, the test will give North Korea a place as a nuclear power alongside the US, Russia, Britain, France, India, Pakistan and China. Israel is also widely believed to have nuclear capabilities.