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.
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.