There is new evidence that North Korea may be preparing for an underground test of a nuclear bomb, U.S. officials told ABC News.
"It is the view of the intelligence community that a test is a real possibility," said a senior State Department official.
A senior military official told ABC News that a U.S. intelligence agency has recently observed "suspicious vehicle movement" at a suspected North Korean test site.
The activity includes the unloading of large reels of cable outside P'unggye-yok, an underground facility in northeast North Korea. Cables can be used in nuclear testing to connect an underground test site to outside observation equipment. The intelligence was brought to the attention of the White House last week.
Even before this most recent intelligence, there has been growing concern within the U.S. government that North Korea has been moving toward a nuclear test. North Korea is believed to have enough nuclear material to build as many as a dozen nuclear bombs, but it has never tested one. A successful test would remove any doubt that North Korea is a nuclear power.
"What does he have to lose?" asked one senior military official, referring to North Korean leader Kim Jong Il.
On July 4, North Korea conducted seven ballistic missile tests, which provoked international condemnation, including a unanimous United States Security Council resolution condemning its actions. A nuclear test, however, would be seen as a much greater provocation than the missile tests. Only seven other nations in the world have ever conducted nuclear tests.
U.S. officials fear a nuclear test could provoke a nuclear arms race in East Asia, forcing Japan and South Korea to develop their own nuclear weapons.
"A nuclear test is going to be alarming and troubling for everyone and would cause a very strong reaction I think from all of North Korea's neighbors," said former National Security Council official Michael Green, now with the Center for Strategic and International Studies.
U.S. officials caution that the intelligence is not conclusive. Last year U.S. spy satellites picked up suspicious activity at suspected test sites in North Korea, leading some to predict an imminent nuclear test, but nothing happened.
Underground nuclear tests are notoriously difficult to detect ahead of time. U.S. intelligence agencies, for example, failed to predict nuclear tests by India and Pakistan in 1998.
Officials say it is possible that North Korea may either be putting on a show for American spy satellites to get attention, or may conduct a nuclear test in an entirely different location.
Some analysts believe Kim Jong Il may feel the only way to be taken seriously is to prove that North Korea is a nuclear power. Officials acknowledge that nobody really knows Kim Jong Il's intentions, but there is a belief among analysts that he is upset about the recent U.N. resolution condemning his missile tests and upset with the Chinese for supporting that resolution.
"It is the view of most in the community that there is a 50-50 chance North Korea will conduct a nuclear test by the end of the year," said one analyst.
Asked what the United States would do in response to a nuclear test, a senior U.S. official told ABC News, "We would try to hermetically seal the hermit kingdom."
The official said the United States would immediately push for sanctions to cut North Korea's ties to the outside world. Another possible option would be a naval blockade of North Korea.
But it is unclear how effective such efforts would be. North Korea is already the most isolated country in the world.