North Korea notified State Department officials after 8 p.m. EDT on Sunday that they planned a nuclear test. President Obama was notified by his national security adviser, retired Gen. James Jones, at midnight, less than an hour after it had taken place, U.S. officials said.
Secretary of State Clinton called the foreign ministers of Japan and South Korea on Monday morning. Obama was to call Japanese and South Korea leaders on Monday night, officials said.
North Korea said it had made significant advances since its 2006 test, and Russian officials estimated that the latest blast was equal to the U.S. atom bomb dropped on Nagasaki. But U.S. officials cast some doubt. A senior administration official noted that in 2006 North Korea "vastly overstated the size of that test." A U.S. official said that U.S. intelligence agencies monitoring the test facility had witnessed significant activity in the days before the explosion. The United States had positioned an array of high-tech equipment to monitor the test, including Pentagon aircraft equipped to collect atmospheric samples of any nuclear plume.
"There are ways of gauging the size and the yield of the explosion," the counter-proliferation official said. "We're still sifting that data right now." The official confirmed that North Korea also tested one or more short-range missiles at the same time it detonated the nuclear device.
The test on Monday measured magnitude 4.52 up from 4.1 from the first test in 2006, U.S. officials said.
U.S. officials say they saw three factors motivating North Korea to conduct the test.
They said the North Koreans have decided that they want nuclear weapons arsenal as a deterrent, and believe that the test will help establish their status as a nuclear state. They believe that the provocative action may help them bring the United States to the negotiating table in a mood to make concessions.
In addition, officials believe that the defiant action reflects the internal political turmoil that is going on in North Korea as powerful groups try to ensure their position when the country's leader, Kim Jong Il, steps down. These groups believe they are better off appearing tough to each other, and to others in North Korea.
