The official, who asked not to be identified, said that first-time nuclear tests historically have been in the several kilotonne range.

"We are aware that there was a sub-kilotonne explosion in North Korea," said the official. "We have not been able to determine at this point whether it was in fact nuclear."

North Korea announced earlier that it had conducted an underground nuclear test, calling it an "historic event" that would contribute to peace and stability on the Korean peninsula.

Other experts around the world reported a seismic event on the Korean peninsula that registered between 3.58 and 4.2 on the Richter scale. Norwegian monitors said their readings indicated an explosion of between one and 10 kilotonnes.

Bryan Whitman, a Pentagon spokesman, said it was "too early" to say whether the blast detected in North Korea was a nuclear device.

"We're evaluating the information that we have as a government. As the president said, we've not been able to confirm it," he said, referring to a statement earlier by President George W. Bush.

Besides seismic data, the US military collects air samples that can be examined for particulates indicative of a nuclear test. North Korean communications and electronic signals also would be targeted and analyzed for clues.

The relatively small size of the explosion could make positive confirmation of a nuclear test more difficult, a defense official said, speaking on condition of anonymity.

He added that the US government had to proceed carefully because of possible North Korean deception.

"I don't think you can rule out the possibility that he's faking out the world," the official said, referring to North Korean leader Kim Jong-il.