WASHINGTON - The U.S. government is officially confirming North Korea's underground atomic test in late May, saying the blast was somewhat larger than the regime's first test, conducted in 2006.

The blast yielded several kilotons, said a U.S. official who spoke on condition of anonymity because the announcement had not yet been made.

Until now the U.S. government has been officially mum on the May 25 test. The official said the national intelligence director was expected to issue a statement Monday confirming the event as a "probable" nuclear explosion.

Independent U.S. analysts have estimated the yield of the atomic explosion to be roughly 4 kilotons. North Korea may now be preparing for its third nuclear test, a show of defiance as the United Nations adopts new sanctions as a result of the May 25 blast.