The "secret weapon" was reportedly launched off the east coast of North Korea, and a test warhead was detonated on Thursday. At that time, she had been at a depth of 80 to 150 meters for more than 59 hours, KCNA reported. The Central Military Commission ordered the maneuver "to warn the enemy of an actual nuclear crisis and to check the reliability of the nuclear force for self-defense."

The military tensions on the Korean peninsula have recently increased drastically. The communist leadership in Pyongyang has repeatedly threatened a military escalation in the region. In the face of North Korea's ongoing provocations, South Korea and the United States have increased their defense cooperation. Most recently, they had started their largest joint military exercises in five years.