Initial environmental samples collected by a U.S. military aircraft detected signs of radiation over the Sea of Japan, possibly confirming North Korea's nuclear test, intelligence officials said yesterday.

Officials said the positive radiation result was consistent with an atomic test and would make it possible to rule out the possibility that Monday's test had been conducted with conventional explosives alone. But intelligence and administration officials were cautious about reaching a conclusion before reviewing all incoming data. "The intelligence community continues to analyze the data," said Frederick Jones, spokesman for the National Security Council. "When the intelligence community has a determination to present, we will make that public."

Earlier detection attempts by the United States, China and South Korea did not pick up any radiation. An intelligence official said additional samples are being collected, and analysts are also taking a harder look at seismic data, satellite photos and communications intercepts.

North Korea, according to U.S. intelligence estimates, has enough plutonium for as many as a dozen nuclear weapons, depending on their level of sophistication. Some analysts have judged Pyongyang's technical capabilities in the nuclear design field to be at a low level. But there are now discussions among top U.S. weapons scientists and analysts whether North Korea may have managed to test a miniaturized warhead or a more sophisticated design than was expected.

"When you look at the rest of their activities, increasing sophistication isn't the first thing you think of, but it hasn't been discarded either," said one nuclear scientist who spoke on the condition of anonymity because of instructions from superiors to avoid talking to reporters about the test.

U.S. intelligence experts say they believe, though cannot prove, that the North Korean blast was the result of a partial implosion of plutonium at the core of the test device. That would mean some of the plutonium failed to implode, but intelligence officials do not claim to know why. The blast yield was less than a kiloton, far smaller than the 20-to-23-kiloton bomb the U.S. military dropped on Japan 60 years ago, and less than the four-kiloton yield the North Koreans told the Chinese to expect in advance of Monday's test.

Several theories about the small yield include the possibility that the device's design was slightly imperfect and thus failed to set off explosive charges simultaneously. Without the simultaneous detonation, the plutonium would fail to fully compress and implode.

If the test was conducted with a miniaturized nuclear device, however, it is possible that the North Koreans were able to conceal any radiation, as they claimed. One analyst suggested that the test may have been conducted in a horizontal tunnel with a vertical drop at the end, thus reducing the chance for radiation to vent into the air.

In an interview with the Hankyoreh, a Seoul-based newspaper, a North Korean diplomat said Tuesday that the test "was smaller than expected, but a small success means a larger success is possible."

He added that the aim was "to possess nuclear weapons," and that it would be possible to take additional measures such as preparing to load nuclear warheads onto missiles.

Even without a definitive conclusion about the test, it set off jitters in neighboring South Korea. A statement signed by 17 former defense ministers and veterans called on the Seoul government to request that the United States redeploy tactical nuclear weapons, which were removed by George H.W. Bush's administration in 1991.

Ironically, it was the initial stationing of U.S. tactical nuclear artillery shells along the demilitarized zone between North and South Korea in the late 1960s that prompted Pyongyang to begin a nuclear program.

The chances of the United States redeploying those weapons are slim; Secretary of State Condoleezza Rice said Thursday that the goal of U.S. diplomacy is to denuclearize the Korean Peninsula.