U.S. and South Korea kick off largest-ever joint military drills amid tension with North Korea.

The United States will mobilize some 17,000 troops, more than twice the number of previous years and deploy a combat aviation brigade and an expeditionary Marine brigade. About 300,000 South Korean personnel will be involved, some 1.5 times more than its average deployment for the annual drills.

The CFC said that the United Nations Command, the unified command structure for the multinational military forces supporting South Korea during and after the 1950-53 Korean War, had informed the Korean People's Army of the DPRK through the truce village of Panmunjom about the exercises dates and their non-provocative nature.

The largest exercises ever staged on the Korean peninsula came after the DPRK tested what it claimed was its first hydrogen bomb on Jan. 6 and followed up with the launch of a long-range rocket, which outsiders see as a disguised test of ballistic missile technology, on Feb. 7.

In an apparent warning to Pyongyang over its nuclear test and rocket launch, the U.S. forces would show off its firepower during the drills by deploying various strategic assets such as a nuclear-powered John C. Stennis aircraft carrier and its attendant fleet, a nuclear-capable submarine and aerial tankers to refuel fighter jets. The B-2 stealth bomber is reportedly under consideration for deployment.

The Key Resolve command post exercise will simulate scenarios in computers to repel the DPRK's invasion like in the past, while applying the new Operation Plan 5015 for the first time to the spring drill.

The OPLAN 5015, which was formally approved by Seoul and Washington in June last year, allegedly encompasses scenarios for pre-emptive precision strikes against the DPRK's nuclear and missile facilities in a contingency with Pyongyang.

Joint amphibious landing maneuvers, called Ssangyong exercise, will also be the largest in scale since it was launched in 2012. It will involve some 3,000 South Korean Marine Corps troops and around 2,000 Navy sailors along with about 7,000 U.S. Marines personnel.

The Ssangyong exercise, which will run from Monday to next Friday, will reportedly strengthen interior operations, rather than landing maneuvers, to practice coming ashore and marching deep into the DPRK areas housing nuclear and missile facilities.

South Korea and the United States have increased the deployment of surveillance assets, including military satellites and reconnaissance aircrafts, against possible provocations from the DPRK which has denounced the war games as a dress rehearsal for northward invasion.

In face of the U.S.-South Korea war games, top DPRK leader Kim Jong Un had ordered nuclear warheads to be placed always on standby for use at any time, according to Friday's report from the DPRK's KCNA news agency.

Kim said the enemies were planning to carry out the "beheading operation" that targets the DPRK leadership and its system. He made the remarks when he guided the test-firing of a new multiple rocket launcher, boosting concerns over the already heightened tensions on the peninsula to be raised further.

On Thursday, the DPRK fired six short-range projectiles into eastern waters in an apparent show of anger over the adoption of new harsher UN Security Council resolutions, which Seoul described as the toughest and most effective non-military measures in seven decades of UN history.

In 2015, Pyongyang launched short-range missiles in a protest against the spring war games on the day when the U.S.-South Korea joint military exercises kicked off.