The U.S. Defense Department says it is still evaluating data from the explosion and small earth tremor in North Korea on Monday to determine whether it was in fact an underground nuclear test, as the North Korean government claims.

Bryan Whitman

Nearly two full days after the explosion in North Korea, U.S. officials, including Pentagon Spokesman Bryan Whitman, cannot say for sure whether it was a nuclear explosion or not.

He said, "We're continuing to assess the event to determine the veracity of the claim made by the North Korean government that they conducted an underground nuclear test."

Whitman says it takes time to analyze the seismic data, conduct atmospheric tests and do other work he cannot discuss to determine what caused the explosion. And he notes that the work is being done in several parts of the U.S. government and by government and private researchers around the world.

Former Defense Department technology official Philip Coyle, who is now with the Center for Defense Information, says the North Korean event is particularly hard to analyze.

He said, "Well, a reason it's taking so long is that it appears to be quite a small nuclear test. Perhaps less than a kiloton, which sounds like a lot but is actually small as nuclear devices go."

The former official, who has 40 years of experience in nuclear testing and evaluation, says it is difficult to isolate the impact of such a small explosion among all the seismic movements detected by sensors in the region.

Japan's Meteorological Agency staff is silhouetted while pointing the seismic wave measured in Japan's various points, Monday, October 9, 2006

But he says once the event is isolated, experts can determine whether it was an earthquake, a conventional explosion or a nuclear detonation. He says a nuclear explosion originates from a much smaller source than an earthquake or conventional explosion of the same magnitude.

In addition, Philip Coyle says efforts have no doubt been made to sample the air near North Korea. But he says any exhaust from such a small test could be very difficult to detect, and North Korea says there was no such exhaust.

Beyond that, Coyle says, intelligence agencies are also likely seeking indications of whether the test was real or not.

"There may be some intelligence that intelligence agencies will be able to get about it, for example, picking up on Internet traffic, voice messages, things like that. In may be that the National Security Agency will have some information also," he said.

Another analyst, former CNN reporter Mike Chinoy of the Pacific Council on International Policy, who has visited North Korea 14 times, says although it is always difficult to know what the country's leaders are thinking, he cannot imagine they would pretend to conduct a nuclear test.

He said, "Without being privy to all the scientific and intelligence information, my own sense is that it would be unlikely for the North Koreans to risk the kind of reaction they're getting around the world, politically, or to take the step of announcing it this way, both the international community and their own people, if they weren't trying to explode something that had a nuclear component."

Some analysts say the North Korean explosion may have been a failed nuclear test, or one that worked only partially.

The Washington Times newspaper quotes unnamed U.S. intelligence officials as saying that is exactly what happened. And a South Korean newspaper quotes a North Korean diplomat as saying the same thing.

That would fit what is known so far about the North Korean explosion. Philip Coyle at the Center for Defense Information says there would not seem to be any reason for North Korea to do such a small test intentionally, but the secretive state might have wanted to keep the world guessing.

"If North Korea's purpose here is to be seen as a major nuclear power, you might think they would want unambiguous test result[s]. But perhaps they meant to make it small. It's hard to know," he said.

On Tuesday, White House Spokesman Tony Snow said the world may never know for sure whether the North Korean explosion was nuclear. But the Pentagon spokesman, Bryan Whitman, said he believes experts will come up with a clear answer.

He said, "We don't have to guess. We don't have to speculate. We will know in time. And when we know, I'm sure you'll know."

Whitman would not say how long the analysis will take, but testing expert Philip Coyle says he thinks there should be a definitive answer within another couple of days.