President Moon Jae-in reaffirmed his commitment to reforming the military and beefing up the national defense capability Sunday during an inaugural ceremony for the new top military officer.

Moon became the first head of state to attend a change of command ceremony held at the Ministry of National Defense compound in Seoul.

"The goal of military reform lies in making an army that wins (battles), is full of patriotism and morale among all ranks of soldiers, and is trusted by the people," the president said at the ceremony as Gen. Jeong Kyeong-doo was sworn in as chairman of the Joint Chiefs of Staff.

"As the commander in chief, I will actively support the military reform," he said. "I will do whatever in my disposal to enhance the military's defense posture against North Korea's nuclear and missile programs and strengthen our defense capabilities."

Jeong, formerly an Air Force chief of staff, replaced Army.

Gen. Lee Sun-jin, who retired after a 42-year service.

The new top military officer issued a stern warning that North Korea will face merciless retaliation for any attack.

He stressed that the current security conditions on the peninsula are "more serious than at any other time" amid the North's fast-advancing nuclear and missile programs.

He said he will focus efforts on establishing a "full range of defense postures" to counter all types of threats.

"If the enemy provokes, (our military) will retaliate resolutely and strongly to make it regret bitterly," Jeong said.

He added deterrence based on combat readiness is a priority.

He also vowed thorough preparations for the deployment of the so-called three-axis defense platform: the Kill Chain pre-emptive strike system, the Korean Air and Missile Defense, and the Korea Massive Punishment and Retaliation scheme. The military plans to put the three elements into operation by the early 2020s.

Jeong, a former fighter jet pilot, is South Korea's first JCS leader hailing from the Air Force in 23 years. President Moon Jae-in's choice is viewed as reflecting his resolve to reform the nation's armed forces long dominated by the Army. Defense Minister Song Young-moo served as the chief of Navy operations.

Jeong and Song, meanwhile, are scheduled to meet separately with Adm. Harry Harris, commander of the US Pacific Command, later in the day.

Harris is visiting South Korea to inspect the allies' Ulchi Freedom Guardian exercise to begin Monday.

The South is fielding around 50,000 troops in the 11-day UFG, a size similar to that of last year. The US, however, has reduced the number of its participating troops to 17,500 from 25,000. The US Forces Korea has not elaborated on the reason.

The UFG is an annual command-post training largely based on computer simulation. (Yonhap)