North Korea said it conducted an underground nuclear test on Monday (October 9), defying a warning from the U.N. Security Council and opening its crippled economy to the risk of fresh sanctions. South Korea put its troops on heightened alert after the announcement, which came just minutes before Japanese Prime Minister Shinzo Abe landed in Seoul for a visit. The move could heighten regional tension and deal a fresh foreign policy blow to U.S. President George W. Bush ahead of mid-term elections. The White House branded the act "provocative" and said it expected the U.N. Security Council to take immediate actions. Long Pyongyang's chief ally, China denounced the "brazen" act, urging it to avoid action that could worsen the situation, and Russian President Vladimir Putin also condemned the test. About 500 South Koreans held an anti-North Korea rally on Monday night, denouncing the North's nuclear test. Protestors burned banners bearing the image of North Korean leader Kim Jong-il, and chanted anti-North Korea slogans. A protest leader said they denounced South Korean government as well as North Korea. "We should denounce the nuclear test for the safety of our lives and our freedom. We should also denounce (South Korean President) Roh Moo-hyun government, because our government let North Korea develop nuclear weapons," said 46-year-old Bong Tae-hong. South Korea's policy toward the North has been an engagement one, called, "Sunshine Policy." North Korea's announcement pushed the dollar to an eight-month high against the yen and helped shove oil above $60 a barrel. South Korea's won fell 1.5 percent to two-month lows and its main stock index tumbled as much as 6 percent. The U.S. Geological Survey said it had detected a 4.2 magnitude tremor in North Korea at 10:35 local time (0135 GMT). The Japan Meteorological Agency said its data showed a tremor took place around Gilju, on the peninsula's northeast coast around 110 km (70 miles) from the Chinese border. Pyongyang's official Korean Central News Agency (KCNA) said there was no leak or danger from its test. "The nuclear test was conducted with indigenous wisdom and technology 100 percent," KCNA said. "It marks a historic event as it greatly encouraged and pleased the KPA (Korean People's Army) and people that have wished to have powerful self-reliant defence capability." Analysts say North Korea probably has enough fissile material to make six to eight nuclear bombs but probably lacks the technology to devise one small enough to mount on a missile. The chief of South Korea's intelligence agency told lawmakers it was possible North Korea would carry out a second test, Seoul's Yonhap news agency quoted one deputy as saying. The lawmaker said Kim Seung-gyu, head of the National Intelligence Service, told a parliamentary committee that unusual signs had been detected at a North Korean town. There was no consensus on the size of the North Korean blast. Gary Gibson of Australia's Seismology Research Centre put it at about one kiloton, the equivalent of 1,000 tons of TNT. Russia's RIA news agency quoted Russian Defence Minister Sergei Ivanov as saying it ranged between 5 and 15 kilotons. The U.S. Air Force dropped a 12.5-kiloton bomb on the Japanese city of Hiroshima in 1945. The U.N. Security Council urged North Korea last week not to carry out a test, warning of unspecified consequences if it did. The United Nation's nuclear watchdog, the International Atomic Energy Agency (IAEA), said Pyongyang's test threatened a global treaty to prevent the proliferation of nuclear weapons. "This reported nuclear test threatens the nuclear non-proliferation regime and creates serious security challenges not only for the East Asian region but also for the international community," the IAEA said in a statement. The Security Council, due to meet on Monday to officially nominate South Korean Foreign Minister Ban Ki-moon as next U.N. secretary-general, was likely to discuss North Korea's move. A U.S. official, speaking on condition of anonymity, said the North had given China a 20-minute warning of its test and Beijing had immediately alerted Washington, Tokyo and Seoul. Seoul and Beijing -- leery of instability on the Korea peninsula -- have previously cautioned against backing the North into a corner, while Tokyo favours a hard line towards Pyongyang. However, all three agree Pyongyang should end its 11-month boycott of six-nation talks on ending its nuclear arms programme. North Korea announced last week its intention to test a nuclear device, saying its hand had been forced by what it called U.S. threats of nuclear war and economic sanctions. But it said it would not be the first to use a nuclear weapon.