U.S. analysts have been quoted as saying that the nuclear weapon exploded Monday may have been much smaller than North Korea intended, and that the success of the test may have been limited.

Ex-Pentagon weapons testing director Philip E. Coyle III was quoted by the New York Times as saying the small size of the test may indicate "what might be described as a partial success or a partial failure."

"As first tests go, this is smaller and less successful than those of the other nuclear powers," Coyle was quoted as saying, adding that if the size of the bomb is determined to be a kiloton or less, "that would suggest that they hoped for more than that and didn't get it."

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The Washington Post said that intelligence officials were looking at four possibilities to explain the size of the blast, the most likely of which appeared to be that only a fraction of the device's had core exploded.

Under those circumstances, the test would still be considered on the whole successful, but it might "lead the North Koreans to conduct additional tests to determine what went wrong."

"A low yield can be a failure in design or it can be bad luck," Michael A. Levi, a nuclear expert at the Council on Foreign Relations told the Post.

"Anything is possible," he said, including simulating a low-yield nuclear explosion by using large quantities of TNT, as the U.S. military had planned to do last summer. "But you don't hear anyone who thinks it's a conventional [explosives] test," he said.

Nonetheless, an unnamed senior government nuclear specialist told the Post, it is difficult to distinguish a nuclear detonation from a conventional one just by looking at seismic data.

"Explosions are explosions," the analyst said. "They push equally out in all directions and that's what you see at a distant seismic station; you just see the explosion force and you can't tell."