< p>BRUSSELS (AP) — NATO's governing body is harshly criticizing North Korea's recent nuclear test, calling it "irresponsible" and a flagrant violation of U.N. Security council actions.

North Korea conducted an underground nuclear test Tuesday, taking an important step toward building a bomb small enough to be fitted on a missile that could reach United States.

"This irresponsible act, along with the December missile launch, poses a grave threat to international peace, security and stability," said a statement from the North Atlantic Council. "North Korea's pursuit of weapons of mass destruction represents continued defiance of the U.N. Security Council and the broader international community."

The statement called on North Korea to immediately cease such "provocative actions." The NATO alliance considers an attack on one of its 28 members as an attack on them all.