US President George W Bush has said North Korea will face "serious repercussions" over its claim to have carried out a nuclear test.
Mr Bush said Washington was working to confirm the claim, but would increase its co-operation with allies on ballistic weapons defence systems.
The comments came as Japan imposed tough new sanctions on North Korea in response to its claim.
The US is leading efforts to get the UN to impose separate measures.
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
Earlier, North Korea's second most powerful leader threatened more tests if the US remained "hostile", in the first comments from a senior North Korean official since the claimed nuclear test on Monday.
The underground test reportedly took place in Gilju in Hamgyong province at 1036 (0136 GMT). Russia is the only country to have confirmed that it was a nuclear explosion.
'Threat to peace'
President Bush told reporters that Washington remained committed to diplomacy, and had no intention of attacking.
But he said the US "reserves all options to defend our friends in the region".
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
Pyongyang's claim "constitutes a threat to international peace and stability", he said.
"In response to North Korea's actions, we're working with our partners in the region and the United Nations Security Council to ensure there are serious repercussions for the regime in Pyongyang."
As Mr Bush spoke, UN Secretary General Kofi Annan urged the US to hold one-on-one talks with North Korea, which Washington has refused to do.
The UN Security Council is debating what multilateral sanctions North Korea should face in response to the claimed test.
It is due to continue discussing a draft resolution of punitive sanctions proposed by the US.
There is agreement in the Security Council that North Korea should face punitive measures.
The US wants the sanctions to be brought under Chapter Seven of the UN Charter, which means they would be mandatory and ultimately enforceable by military means.
But China, Russia and South Korea have expressed varying degrees of opposition to such a resolution.
The Japanese sanctions, announced by Chief Cabinet Secretary Yasuhisa Shiozaki, will come into effect following a formal cabinet meeting on Friday.
They include banning all North Korean imports, stopping its ships entering Japanese waters and new restrictions which will prevent almost all North Koreans from entering Japan.
Trade between the two countries was worth $180m (£97m) last year, although it has been falling for several years as political relations between the two countries deteriorated.