A senior military official told ABC that an unidentified US intelligence agency had recently observed "suspicious vehicle movement" at a suspected North Korean test site.

The activity included the unloading of large reels of cable outside an underground facility called Pungyee-yok in northeast North Korea, it said.

ABC said cables can be used in nuclear testing to connect an underground test site to observation equipment.

It said the intelligence had been brought to the attention of the White House last week but cautioned that it was "not conclusive."

An imminent nuclear test was predicted in North Korea last year but no test occurred.

ABC said underground nuclear tests are "notoriously difficult" to detect ahead of time and noted that the United States had failed to predict nuclear tests by India and Pakistan in 1998.

North Korea claims to have built nuclear weapons and sparked international alarm last month by test-firing seven ballistic missiles.

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