WASHINGTON — U.S. intelligence agencies were still collecting data Monday following North Korea's announcement that it had carried out a nuclear test, but analysts expressed little doubt that the regime had exploded such a device.
"There is strong reason to believe that North Korea did in fact conduct a nuclear test," said a U.S. counter-proliferation official who requested anonymity. "We're still reviewing the data to see how large it was, but there's no reason to doubt the North Korean announcement."
The 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 North Korean nuclear program remains the most urgent military and diplomatic problem in Asia, and U.S. military officials had ramped up attention paid to the threat after the regime tested a long-range rocket last month.
"We had been anticipating this for some time both through their rhetoric and activity we had seen going on," said a military official who also requested anonymity given the sensitive nature of the issue. "We were not taken by surprise."
U.S. officials had not reached a conclusion on the explosive power of the blast. Seismic data showed a magnitude of 4.5, up from 4.1 from a test North Korea conducted in 2006. Using that data, some experts said the explosion would have been slightly larger than the previous test but far smaller than those from the first atomic bombs exploded over Hiroshima and Nagasaki.
Russian officials reported that their forces had monitored a much stronger blast.
"The special control service of the Defense Ministry of the Russian Federation registered an underground nuclear explosion equivalent to 10-20 kilotons in the territory of the Democratic People's Republic of Korea," Col. Alexander Drobyshevsky told Interfax.
Meteorologists were monitoring radiation in the Russian Far East, but had not registered any change in background radiation by late afternoon Monday, the Russians said.
U.S. analysts voiced skepticism over the report, which would suggest a major leap for North Korea's nuclear program.
Although U.S. military intelligence knew the test was imminent and the North Koreans gave the American mission to the United Nations a short warning, U.S. military forces were not put on a higher alert status before or after the explosion.
The Pentagon did not want to appear to be responding militarily to the provocations, according to the military official.
"Diplomacy needs to be the lead effort here," said the official. "That is where the focus is right now, not on any sort of military response."
Times staff writer Megan K. Stack in Moscow contributed to this report.