South Korean and U.S. military authorities had been observing signs of North Korean preparations for another nuclear test in Kilju, North Hamgyong Province where it conducted the first in October 2006.
According to a source, the two countries monitored vehicle and personnel movements there through the U.S.' KH-12 reconnaissance satellite. They also observed construction work to expand an underground mine, where it would conduct a nuclear test, and the erection of a building in a nearby area, a senior South Korean government official said.
A government official said, "It was hard to find out exactly when it would be possible to conduct a nuclear test due to the difficulty of making predictions about an underground test. But we'd judged from early this month that since it made the threat in April, the North had finished preparations to conduct a test in the near future if it wanted."
U.S. reconnaissance satellites can detect signs of impending long-range missile launches since they require a vertical launch pad and are fed with liquid fuel, but an underground nuclear test is less easy to predict.
South Korean and U.S. military authorities speculated that the North would delay a second nuclear test a little in consideration of the chaotic situation in South Korea after the death of former president Roh Moo-hyun, but that was evidently no consideration.