China and Russia have delayed a vote on a new UN resolution, drafted by the US, that calls for sanctions on North Korea over its claimed nuclear test.
North Korea has refused to back down in the nuclear stand-off
The US had wanted to put its new draft of a resolution to a vote on Friday.
The draft targets the North's missile and nuclear programmes under the UN's Chapter Seven - which makes sanctions mandatory and may allow using force.
China has concerns over the enforcement of a resolution and, with Russia, wants more time to consider the US draft.
Following talks at the UN Security Council on Thursday, China is to send a top envoy to discuss the matter with US President George W Bush.
The envoy is expected to travel to Moscow on Saturday for further talks.
Western diplomats believe China's support for the draft resolution is essential if tough action is to be taken against North Korea, says the BBC's UN correspondent, Laura Trevelyan.
North Korea's underground test reportedly took place at 1036 (0136 GMT) on Monday in Gilju in north-east Hamgyong province.
US SANCTIONS PROPOSAL
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
Russia is the only country to have confirmed that it was a nuclear explosion, amid speculation the test was not wholly successful.
South Korea said on Thursday that it had not detected any abnormal radioactivity levels on its soil after its northern neighbour's alleged nuclear test.
On Friday Japan's cabinet approved fresh sanctions on North Korea, including banning all imports from the country.
Pyongyang said earlier it would take "strong countermeasures" against Japan if the sanctions were enforced.
N KOREA NUCLEAR PROGRAMME
Believed to have 'handful' of nuclear weapons
But not thought to have any small enough to put in a missile
Could try dropping from plane, though world watching closely
There is agreement in the UN Security Council that North Korea should face punitive measures, but countries are divided over how tough these should be.
The US wants the sanctions - which would also target luxury goods - to be brought under Chapter Seven of the UN Charter.
This means they would be mandatory and ultimately enforceable by military means.
The draft resolution also includes a clause allowing nations to ban the entry or transit of people believed to support Pyongyang's weapons programme, reports say.
But China, Russia and South Korea have expressed varying degrees of opposition.
China is also reported to be unhappy about authorising inspections of cargo vessels moving in and out of North Korea in order to find weapons-related material.