The National Intelligence Service says North Korea has performed a nuclear test at an underground tunnel in a mountain in Hamgyeong Province.

The intelligence agency said that the nuclear test was conducted around 10:35 a.m. on Monday at a site near Musudanri, North Hamgyeong Province where Pyongyang had test-fired its long-range Taepodong-Two missile in July.

It went on to say that, considering the mountain's three-hundred-60 meter-height, the test site was not a vertical tunnel but one that was crosscut.

Meanwhile, Reuters quoted a U.S. official as saying that, 20 minutes before the test, Pyongyang notified Beijing which in turn promptly informed Seoul, Washington and Tokyo.