SEOUL, South Korea – North Korea defiantly declared Monday that it carried out a powerful underground nuclear test — reportedly many times larger than a 2006 test — in a major provocation in the escalating international standoff over its rogue nuclear and missile programs.
The regime also test-fired a short-range, ground-to-air missile Monday from the same northeastern site where it controversially launched a rocket last month, the Yonhap news agency reported, citing unnamed sources. That liftoff, widely believed to be a cover for a test of its long-range missile technology, drew censure from the U.N. Security Council.
North Korea, incensed by the condemnation of the April 5 rocket launch, had warned last month that it would restart it rogue nuclear program, conduct an atomic test and carry out long-range missile tests.
On Monday, the country's official Korean Central News Agency said the regime "successfully conducted one more underground nuclear test on May 25 as part of measures to bolster its nuclear deterrent for self-defense."
Defense officials in Russia, which shares a short border with North Korea, confirmed a nuclear test in northeastern North Korea, the ITAR-Tass news agency said.
President Barack Obama said a nuclear test would constitute an act of "blatant defiance" of the U.N. Security Council and a violation of international law.
"These actions, while not a surprise given its statements and actions to date, are a matter of grave concern to all nations," he said, calling for international action in a statement from Washington. "North Korea's attempts to develop nuclear weapons, as well as its ballistic missile program, constitute a threat to international peace and security."
"If North Korea carried out a nuclear test, it would clearly violate U.N. Security Council resolutions," chief government spokesman Takeo Kawamura told reporters in Tokyo. "We will definitely not tolerate it."
Japan will request an emergency U.N. Security Council meeting to discuss North Korea, Vice Foreign Minister Mitoji Yabunaka said, according to the Kyodo news agency.
North Korea did not say where or when the test took place. But seismologists from the U.S., South Korea and Japan reported activity shortly after 9:50 a.m. in a northeastern area where North Korea conducted its first nuclear test in 2006.
An emergency siren sounded in the Chinese border city of Yanji, 130 miles (200 kilometers) northwest from the test site. A receptionist at Yanji's International Hotel said she and several hotel guests felt the ground tremble.
The Japan Meteorological Agency measured the seismic activity at magnitude-5.3, quake expert Gen Aoki said. The Korea Institute of Geoscience and Mineral Resources in Seoul reported seismic activity in Kilju in North Hamgyong Province — the same area where North Korea carried out a nuclear test in October 2006.
North Korea boasted that Monday's test was conducted "on a new higher level in terms of its explosive power and technology of its control" than one carried out in 2006.
The force of the nuclear blast was 10 to 20 kilotons, Russian Defense Ministry spokesman Alexander Drobyshevsky told the state-run RIA-Novosti news agency.
An explosion of that size would be many times more than North Korea managed in 2006. U.S. intelligence officials said the 2006 test measured less than a kiloton; one kiloton is equal to the force produced by 1,000 tons of TNT.
Radiation levels, tested after the blast, were normal, RIA-Novosti cited a spokeswoman for the meteorological official in the Primorye region as saying.
The reported test-fire of the short-range missile took place at the Musudan-ri launchpad on North Korea's northeast coast, Yonhap said. Sources described it as a ground-to-air missile with a range of 80 miles (130 kilometers).
Japan's coast guard had said Friday that North Korea warned ships to steer clear of waters off a coastal city near the missile launch site, suggesting Pyongyang was preparing for a missile test. Yonhap also had reported brisk activity along the northeast coast over several days last week.
South Korean troops were on a high alert but there was no sign North Korean soldiers were massing along the heavily fortified border dividing the two nations, according to an official at the Joint Chiefs of Staff who spoke on condition of anonymity, citing agency policy.
The two Koreas technically remain at war because their three-year conflict ended in a truce, not a peace treaty, in 1953.
North Korea is believed to have at least a half-dozen atomic bombs. However, experts say North Korean scientists have not yet mastered the miniaturization technology for mounting a nuclear device onto a long-range missile.
The 2006 test, which drew tough U.N. sanctions, prompted North Korea's neighbors and the U.S. to push for a pact offering Pyongyang energy aid in exchange for dismantling its nuclear program. North Korea signed the accord in February 2007, but the process has been stalled since last July.