China has called for "appropriate" UN action over North Korea's claim to have carried out a nuclear test on Monday.
North Korea's claim sparked angry protests in the South
Beijing - traditionally Pyongyang's closest ally - said it had not ruled out UN sanctions but that military action was "unimaginable".
The UN Security Council is considering a draft resolution that proposes strict financial and trade sanctions.
The US ambassador to the UN said while the US would not rule out using force, it was seeking a diplomatic solution.
The South Korean Prime Minister, Han Myung-sook, said Seoul would not support a resolution including a threat of 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.
Chinese foreign ministry spokesman Liu Jianchao said: "This will no doubt have a negative impact on China and North Korea's relations."
He said the UN should take "appropriate action" but added that China was still considering the nature of that action.
However, when asked what China thought of the possibility of military action, Mr Liu told a news conference: "I think this is an unimaginable way."
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."
Meeting in New York, the UN Security Council has strongly condemned North Korea over its claimed test.
On Tuesday the Security Council will continue to weigh up options for punitive action, and is considering a 13-point draft resolution proposed by the US, 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
US ambassador to the UN John Bolton said that while the US would never rule out the use of force, they were seeking a diplomatic solution to the crisis through the UN.
"What we're seeking in this resolution in the wake of their nuclear test, is to strengthen... sanctions, make them more comprehensive, make it harder - hopefully impossible - for North Korea to proceed down the road to becoming a nuclear power with delivery capability," he said.
"That would involve a range of things, cutting off their access to sensitive technology and materials, going after the financial network that exists to help them fund this sort of activity, and a range of other things as well."
The US 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.
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."
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.
Scientists in South Korea say they believe the North's claim is genuine, but are trying to get further confirmation.
The size of the bomb is still uncertain, with estimates varying from 550 tons of destructive power to as much as 15 kilotons.
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.
But correspondents say that just because Pyongyang has nuclear capabilities, it does not necessarily have a fully-fledged nuclear bomb, or a warhead that it can deliver to a target.