The U.N. Security Council last Friday urged North Korea not to carry out a test, warning Pyongyang of unspecified consequences if it did. South Korea's presidential Blue House said a tremor of magnitude 3.58 to 3.7 had been detected in North Korea. However, the U.S. Geological Survey, said it detected a 4.2-magnitude tremor at 10:35 p.m. EDT, which could mean the device was potentially deadlier than initially believed. Australia also said there was seismic confirmation that North Korea conducted a nuclear test. In the meantime, South Korea suspended a scheduled aid shipment as a response to the nuclear test, the Yonhap news agency reported earlier this morning. The South had scheduled to ship 4,000 tons of cement to North Korea on Tuesday, but decided to delay it "under the current circumstances," Yonhap quoted an unidentified Unification Ministry official as saying. Officials from Washington to Seoul have been warning of a possible arms race even before North Korea said it fulfilled its threat to join the elite club of nuclear powers. South Korean Vice Foreign Minister Yu Myung-hwan had said a test could give Japan a pretext to go nuclear, triggering countermoves by suspicious Asian neighbors in a cascade that upends regional security.

"There's no equalizer like the bomb," said Peter Beck, head of the Seoul office of the International Crisis Group think tank. He told FoxNews that "It's safe to say it will lead to an arms race -- will push all the governments in the region to increase defense spending." Defense Secretary Donald H. Rumsfeld warned Thursday that allowing North Korea to test a bomb would provoke far-reaching fallout. "The lack of cohesion and the inability to marshal sufficient leverage to prevent North Korea from proceeding toward a nuclear program ... it will kind of lower the threshold, and other countries will step forward with it," Rumsfeld said. The current North Korean nuclear standoff dates to 2002, when the United States accused North Korea of conducting a secret nuclear program in violation of a 1994 agreement. Senior U.S. officials said the United States is consulting with allies around the world and would push for sanctions Monday at a 9:30 a.m. meeting of the U.N. Security Council in New York.