WASHINGTON – A growing chorus of intelligence sources was leaning Tuesday toward the conclusion that North Korea's claim that it successfully detonated a nuclear weapon last Sunday may be more bluster than blast.
One U.S. intelligence official told FOX News that "there are some very, very strong factors to assume that it was a nuclear test, perhaps less than what the North Koreans were hoping for, but that's still unconfirmed.
"The working assumption remains that it was a nuclear test," the official said, cautioning that the conclusions were not firm.
The Washington Times, however, quoting anonymous intelligence officials, reported Tuesday that the second part of what appears to be a two-part explosion likely did not occur. North Korean scientists were believed to be using massive amounts of conventional explosives to try to set off a more powerful atomic chain reaction that results in a nuclear blast.
It likely was those conventional, non-nuclear explosives that registered the reported seismic measurements, and not a nuclear explosion, which experts said should have registered some 15 to 20 times more powerfully, the newspaper reported.
Intelligence and Defense Department officials confirmed to FOX News that the test could have been a dud, but they also advised against concluding too quickly that it was a non-nuclear event.
U.S. military aircraft equipped with radiation-detection instruments were monitoring the Korean peninsula for nuclear activity at the time of the reported explosion. Intelligence officials have said that it should be apparent within hours after a nuclear test whether it was successful, and the longer time passes without that indication the greater the likelihood that it was not.
At the Pentagon, military officials said that while they are working under the assumption that a small nuclear explosion did happen, they were "continuing to assess the event."
"Things like seismographic data," Pentagon spokesman Bryan Whitman said. "There is the possibility of particulate fallout that is detectable and then there is a variety of other intelligence means to determine the veracity of the allegation.
"There will be a period of time where more information is being gathered and at some point we will know more about this test," he said.
While officials are not yet ready to publically declare North Korea's "test" was a dud, the evidence appears to be mounting that if the regime of Kim Jung Il tested a nuclear device, the hoped for results fell short.
An unidentified diplomat at the North Korean Embassy in Beijing reportedly admitted to South Korea's Hankyoreh newspaper that the test was "smaller in scale than expected."
The diplomat claimed Pyongyang has the ability to detonate a more powerful device, and North Korea could take "additional measures" following Sunday's test.
"The success in a small-scale (test) means a large-scale (test) is also possible," he said in comments posted on the newspaper's Web site.
Rep. Heather Wilson, R-N.M., told reporters Tuesday that she has seen a declassified brief that indicates the explosion was a "sub-kiloton" event, meaning that the explosion did not amount to the multi-kiloton power expected from successful nuclear detonation.
White House spokesman Tony Snow, weighing in on the matter during his White House briefing, indicated skepticism within the administration.
"You could have something that is very old and off-the-shelf here, as well, in which case they've dusted off something that is old and dormant," he said.
Snow told reporters that the question of whether it was successful or not are moot, "because you still have a deliberate provocation. ... Nuclear or not, they made a statement ... that they had fired one," giving countries opposed to the action a reason to band together.
"As with anything else with North Korea, it's opaque. We don't know. That is one of the reasons why it's been very difficult for anyone at this juncture — and this would include all of our allies — to assess precisely what took place the other day," Snow said.
"The real point of emphasis for us is, let's figure out diplomatically how to make sure that the number of nukes they have is zero," he added.
Meanwhile, North Korea's tests have caused fears that international nuclear non-proliferation pacts might be in tatters.
Officials told FOX News that countries including Turkey, Egypt, Saudi Arabia and Nigeria are reconsidering their positions on nuclear weapons programs.
The test also has raised new fears that North Korea — which is widely believed to be a heavy participant in the global underground economy because of already heavy sanctions — could be poised to begin trading nuclear weapons and materials with terror networks like Al Qaeda.
CountryWatch: North Korea
FOX News' Catherine Herridge, Nick Simeone and Mike Emanuel contributed to this report.