NEW : ` ` It 's part and parcel of their threats to engage in more provocations , '' says U.S. official . Last month , the U.N. Security Council voted to tighten sanctions on Pyongyang , after the North launched a satellite aboard a long-range rocket in December . The Pentagon has been warning for months that North Korea is preparing a missile attack on the United States or its allies - but it was unclear if Pyongyang had actually tested such an attack. North Korea has conducted two previous nuclear tests , in 2006 and 2009 .
NEW : ` ` It 's part and parcel of their threats to engage in more provocations , '' says U.S. official . The Pentagon has been warning for months that North Korea is preparing a missile attack on the United States or its allies - but it was unclear if Pyongyang had actually tested such an attack. North Korea has conducted two previous nuclear tests , in 2006 and 2009 .