Kim Jong-un missile
North Korea launched a missile into the sea Friday, April 1, 2016. In picture: North Korean leader Kim Jong-un watches a long range rocket launched into the air in this still image taken from KRT footage and released by Yonhap on Feb. 7, 2016.Reuters

South Korea and the United States started their annual joint military exercise Monday in the face of a threat from North Korea that warned of a "preemptive" nuclear strike. South Korean officials reportedly said that the exercise was focused on improving the capability to launch strikes targeting the top leadership of the Kim Jong-un regime. 

North Korea issued a warning Monday threatening to launch "pre-emptive nuclear strike of justice" if the U.S. and South Korea persisted with the military drill. 

"The army and people of the DPRK will launch an all-out offensive to decisively counter the U.S. and its followers' hysterical nuclear war moves," North Korea's National Defense Commission said, according to the state KCNA news agency. Democratic People's Republic of Korea (DPRK) is the official name of the isolated country. 

Despite the threat, 17,000 U.S. troops and 300,000 South Korean troops began participating in the military drill Monday, according to South's Yonhap news agency, which termed it as their "largest-ever annual joint military exercises." The agency cited officials stating a "special focus on bolstering the allies' wartime capability to launch precision strikes on North Korea's top leadership."

The U.S. will deploy aircraft carrier USS John C. Stennis (CVN-74) and a nuclear-powered submarine, as well as the B-2 stealth bomber, as per the report. 

Much of the military drill is likely to be focused on the North Korean threat, with South Korea and the the U.S. having reportedly coined an operational plan as "4D" to "detect, disrupt, destroy and defend" against North Korea's nuclear weapons. 

North Korea had last year reacted to the drills by firing short-range ballistic missiles. The country has recently been slapped with its heaviest set of sanctions by the United Nations for conducting a nuclear test in January and a missile launch last month.