Recent images taken by US spy satellites reportedly show activity at a suspected North Korean test site at Gilju, in the north-east of the country.
Analysts see Kim Jung Il as having a keen ability to escalate tension in increments
The images show excavation and some construction which, according to a US defence official, could be preparations for an underground nuclear test.
But the official also warned that the US intelligence community had not concluded that a nuclear test was imminent.
"It's tough to say. They could be [preparations], they could not be. And there's disagreement within the community over the significance of this," he said.
He also warned that the activities could be deliberately staged by the North Koreans "as a ruse".
The North Koreans have "significant information operations", he said.
His implication was that the North Koreans could have staged the excavation to simulate preparations for a test purely for the benefit of spy satellites.
The logics behind such a move are difficult to fathom.
But by encouraging the perception that a test is imminent, North Korean leader Kim Jong-il may calculate that he increases tension, and thus his leverage with the international community.
If North Korea does conduct an underground test, the world will know immediately.
The reverberations from the test will be picked up by seismic measuring stations in the region. Intelligence agencies will detect chemical and radiological emissions through ship and airborne sensors.
Analysts with links to US intelligence say it is still not known for sure whether North Korea has the ability to set off a nuclear explosion - simply because they never have.
But it is virtually certain, they say, that North Korea has produced plutonium through the reprocessing of spent fuel rods at its Yongbyon reactor complex.
And that has led to a working assumption in US intelligence that the North Koreans have produced a nuclear weapon.
However, in South Korea, China and Russia, say the analysts, there is great scepticism that North Korea has a working bomb.
There are fears North Korea may be about to test a nuclear bomb
In and of itself, an underground nuclear test would not confirm that North Korea has a deployable nuclear weapon - much less one that is miniaturised, configured to fit atop a missile, and programmed to explode when and where it is intended to.
"[Kim Jong-il] might be able to detonate a wad of plutonium to create a nuclear reaction, but that is a long way from a bomb you can put on a [missile] and visit on Los Angeles," said one analyst.
Nonetheless, a nuclear test by North Korea would have profound political implications.
"It would encourage a major policy rethink in the US," said the analyst. "And China will have absolutely no sense of humour."
The response from the rest of the world is also likely to be robust.
It could include referral to the United Nations Security Council for sanction, as well as unilateral action by the United States.
Certainly it would destroy hopes of a diplomatic settlement of the international row over North Korea's nuclear programme.
And it would eliminate any possibility of North Korea acquiring the food and energy it seeks.
A strong adverse response by China - a supplier of grain, fuel and a measure of diplomatic succour - could prove terminal to Kim Jong-il's regime.
Such uncertainty would seem to militate against the possibility an early nuclear test. But Mr Kim is nothing if not counterintuitive in the way he proceeds.
And nobody in Washington is ruling out the possibility that he plans to test.
Yet analysts here see Kim Jong-il as possessing a keen ability to escalate tension by small increments when it suits him to do so, always stopping short of provoking a concerted international response.
"He slices the salami very thin indeed," said one.
The claim by the North two months ago that it does indeed possess nuclear weapons was just such an escalation. So was the recent missile test.
Perhaps the activity at Gilju is, too.