North Korea has dismissed reports that it is to conduct underground nuclear weapons testing and said the allegations had been "cooked up" by Washington.
However, there was no outright denial that testing could take place in the future.
Media reports have said spy satellites show North Korea has apparently stepped up activity in its northeastern region of Kilju. The area has been suspected of being where the North would conduct a test, US and South Korean officials have said.
They claim data from spy satellites indicates cranes, trucks and other heavy equipment are in the area digging holes and conducting other activity that increase the prospects of an imminent test.
The United States has said it may take North Korea to the UN Security Council, where Pyongyang could face possible sanctions, if the North stays away from six-party talks aimed at ending its nuclear ambitions.
The official North Korean news agency KCNA commented: "Let the United States do whatever it wants. That's our bold stance."