WASHINGTON (CNN) -- The U.S. intelligence community believes that North Korea tested a nuclear device last month with an explosive yield of several kilotons, considerably more powerful than its first test nearly three years ago.
In a brief statement, the office of Director of National Intelligence Dennis Blair said, "The U.S. Intelligence Community assesses that North Korea probably conducted an underground nuclear explosion in the vicinity of P'unggye on May 25, 2009. The explosion yield was approximately a few kilotons. Analysis of the event continues."
May's nuclear test was the second conducted by the North Koreans. The first in October 2006 had a yield of approximately a half-kiloton. At the time, U.S. officials and independent experts considered the test somewhat of a failure.
U.S. and international nuclear experts had estimated that the latest test was in the 3- to 4-kiloton range. Former nuclear weapons inspector David Albright, the director of the Institute for Science and International Security, said the test shows that the North Koreans are "pushing their program along" and that they have "made progress."
By comparison, the first atomic bombs developed by the United States produced an explosion equivalent to about 19 kilotons, or 19,000 tons of TNT, according to the U.S. Air Force. The U.S. atomic bombs dropped on the Japanese cities of Hiroshima and Nagasaki in 1945 each generated explosions of more than 15 kilotons of TNT.
A U.S. official said last week that there are indications the North Koreans "may be preparing" for another nuclear test. Albright said further testing could "significantly advance [North Korea's] nuclear weapons craft."
Over the past several months, North Korea has also tested a number of short-, medium- and long-range missiles. The U.S. official said North Korea is continuing to try to improve its technology on the full range of weapons.
"The more you conduct tests, the more you learn," said the official.