North Korea says it carried out an underground nuclear test Tuesday . It is the third nuclear test Pyongyang has carried out since 2006 . `` That is essentially unknown , or known by a few people inside North Korea . '' It comes after new U.N. sanctions for North Korea 's December satellite launch . The U.S. and South Korean militaries conducted a joint drill in waters off the western coast of Seoul on Wednesday, with some 300 troops participating.
North Korea says it carried out an underground nuclear test Tuesday . It is the third nuclear test Pyongyang has carried out since 2006 . It comes after new U.N. sanctions for North Korea 's December satellite launch . The U.S. and South Korean militaries conducted a joint drill in waters off the western coast of Seoul on Wednesday, with some 300 troops participating.