U.S. private experts are forecasting an imminent third nuclear test by North Korea.

A number of U.S. experts on Korean affairs and private think tanks have disclosed satellite images showing the North Korean region where two nuclear tests were previously conducted. They say the North could conduct a nuclear test as early as within a week.

U.S. expert on Korean Peninsula affairs, Joel Witt, told the Associated Press that digging activity for nuclear tests has been detected at the North's previous nuclear site in Gilju, North Hamgyeong Province. Citing the satellite images, he says the North has clearly been preparing for a nuclear test over the past few months.

However, the U.S. government is exhibiting caution, noting the possibility of a nuke test is always present but concrete information has not been confirmed.

One diplomatic source said Sunday, local time, that a nuke test could be imminent considering the activity detected in the recent satellite photos and the fact that North Korea conducted its first and second nuclear tests two months and six weeks, respectively, after it test-fired missiles.