By Sam Kim
SEOUL, May 25 (Yonhap) -- North Korea said Monday it successfully carried out a second nuclear test to bolster the "might" of its arms, less than two months after it fired a rocket that neighbors fear could be turned into a ballistic missile capable of hitting the U.S.
Its official media said the test was "successful" and "helped satisfactorily settle the scientific and technological problems" that may have arisen after it conducted its first test in October 2006.
South Korea said its weather agency detected tremors of "an artificial earthquake" measuring at 4.5 on the Richter magnitude scale in the northeastern part of the communist neighbor at 9:54 a.m.
A source, who declined to be named, said North Korea coupled the detonation with a test of a short-range missile from a coastal base where the communist regime conducted its April 5 rocket launch.
The U.S. refrained from confirming the North Korean announcement, but President Barack Obama called Pyongyang's action "a threat to international peace."
"The danger posed by North Korea's threatening activities warrants action by the international community," he said.
Russia said the "nuclear explosion" appears to have had a force of up to 20 kilotons, according to its state media. U.S. intelligence had said that the October 2006 test had a yield of less than one kiloton. A nuclear test is considered successful when it yields between five and 15 kilotons.
"The current nuclear test was safely conducted on a new higher level in terms of its explosive power and technology of its control," the North's Korean Central News Agency said, monitored in Seoul.
The test, which South Korea said appears to have taken place in Poongkye-ri, North Hamgpyong Province -- the site of the 2006 blast -- was aimed at "further increasing the power of nuclear weapons and steadily developing nuclear technology," the report said.
"The nuclear test is a serious threat to peace and stability on the Korean Peninsula," South Korean presidential spokesman Lee Dong-kwan said in a government statement, calling it "intolerable" and "a serious challenge to the international regime on nuclear non-proliferation."
In a meeting with his Japanese counterpart Hirofumi Nakasone, South Korean Foreign Minister Yu Myung-hwan agreed to push for a U.N. Security Council emergency meeting, while the defense ministry in Seoul placed its 655,000 troops on a heightened alert.
Chinese Foreign Minister Yang Jiechi, in his talks with Yu, urged that regional peace and stability be given top priority in dealing with North Korea's latest challenge. Diplomatic sources said North Korea had given Beijing prior notice of the nuclear test.
Since the U.N. Security Council condemnation of its rocket launch, North Korea has threatened additional nuclear and missile testing, vowing to toughen its "nuclear deterrent."
Pyongyang claims it put a satellite in orbit with the launch while Seoul and Washington said nothing entered space, calling it as a provocative test of long-range ballistic missile technology.
South Korean and U.S. experts downplay the ability of the communist regime to tip its intercontinental ballistic missiles with nuclear bombs.
Protesting the international objection to its rocket launch, North Korea has vowed to jettison the six-nation denuclearization-for-aid talks and expelled outside monitors from its nuclear facilities.
The North is believed to have plutonium enough to create up to six nuclear bombs. It has vowed to restore its Yongbyon nuclear reactor, which had been undergoing disablement under a pact signed by the two Koreas, the U.S., Japan, Russia and China.
Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, said North Korea is stepping up its pressure on Washington to squeeze "maximum" concessions from it.
North Korean leader Kim Jong-il also knows that U.N. sanctions, imposed after the 2006 test, would have little economic impact on his already isolated country, he said.
"Kim is following his roadmap under meticulous calculations," he said. "After the sanctions and temporary condemnations, he is looking at maximizing profits North Korea may get by holding nuclear disarmament talks with the U.S."
Yang added North Korea appears to be expecting increased internal military unity as a result of the nuclear test.
South Korea's stock and currency markets briefly tumbled after the North Korean announcement, but recovered most of their earlier losses.
The benchmark Korea Composite Stock Price Index (KOSPI) fell 2.85 points, or 0.2 percent to 1,400.90. The local currency closed at 1,249 won to the U.S. dollar, down 1.6 won from Friday's close.
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