US formally confirms North Korea's second nuclear test

North Korea tested a nuclear device last month with an explosive yield of several kilotons, considerably more powerful than its first test in 2006, the US has confirmed.

"The US Intelligence Community assesses that North Korea probably conducted an underground nuclear explosion in the vicinity of Punggye on May 25, 2009. The explosion yield was approximately a few kilotons. Analysis of the event continues," the office of Director of National Intelligence Dennis Blair said in a brief statement.

May's nuclear test was the second conducted by the North Korea. The first in October 2006 had a yield of approximately a half-kiloton.

Blair's statement comes amid concerns that North Korea may be preparing for yet another nuclear test. It also came on the eve of South Korean President Lee Myung-Bak's talks with his American counterpart Barack Obama about North Korea.

Lee was expected to ask Obama for explicit security guarantees after the North Korean nuclear test and a series of provocative missile tests.

US 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 programme along" and that they have "made progress."

On June 12, the UN Security Council unanimously decided to tighten financial and other sanctions in response to the latest nuclear test.

Pyongyang reacted sharply the next day with defiant vows to build more bombs and to start on a new weapons programme based on uranium enrichment.