(ATTN: UPDATES details, background; CHANGES slug)
SEOUL, Feb. 12 (Yonhap) -- North Korea is believed to have conducted its third nuclear test on Tuesday, South Korean officials said, after an artificial quake across the heavily armed border strongly suggested that Pyongyang had exploded a nuclear device.
A magnitude-5.0 tremor occurred at 11:58 a.m. and was centered in Kilju County in North Korea's northeast, where the country's Punggye-ri underground nuclear test site is located, officials said.
North Korea gave advance notice to the United States and China on Monday that it was going to conduct a nuclear test, officials said.
"A series of signs have been detected since yesterday that showed North Korea was going to go ahead with a nuclear test," the official said. "There have been past cases where North Korea gave advance notice to the U.S. and China before a nuclear test."
North Korea's state media have not release any comment yet.
The suspected blast, if confirmed, would mark the North's third nuclear test.
Pyongyang conducted its first nuclear test in 2006 and the second one in 2009.
South Korean President Lee Myung-bak has convened an emergency meting of the National Security Council. Lee has warned the North of "grave consequences" if the regime forges ahead with a nuclear test.
North Korea has threatened to conduct an atomic test after a U.N. Security Council resolution condemned its long-range rocket launch in December and toughened sanctions on the isolated nation.
Pyongyang has claimed the launch was part of a peaceful space program.