President Park Geun-hye on Sunday appointed Defence Minister Kim Kwan-jin to lead the presidential National Security Office, her spokesman said, in a show of her resolve to deal sternly with increasing North Korean threats.
Han Min-koo, former chairman of the Joint Chiefs of Staff, was also nominated as Kim's successor.
The announcement came about 10 days after then-national security chief Kim Jang-soo and National Intelligence Service director Nam Jae-joon stepped down in the aftermath of the Sewol ferry disaster.
Kim Kwan-jin will take office immediately, with his new position not subject to a parliamentary hearing. He will also continue ministerial duties until Han is approved by the National Assembly.
"The nomination is to prevent any vacuum in national security and strengthen the security posture in the face of North Korea's constant provocations and threats that are jeopardizing the people's safety and national security," presidential spokesman Min Kyung-wook told reporters.
Kim, 65, vowed to "devote" himself to solidifying national security and building the foundation for peaceful unification, a Defence Ministry spokesman quoted him as saying.
A hard-line, reform-minded former Army general, Kim was appointed to the minister post shortly after the North's attack on a South Korean border island in 2010 and retained under the Park administration in 2013, becoming one of the country's longest-serving defence chiefs.
He shot to fame for his upright posture and a series of remarks warning of strikes and other punishment against North Korea in the event of its provocation.
Pyongyang has been pouring out scathing invectives against him, calling him "the No. 1 target of grave-weeding," "first-class warmonger" and "the boss of puppet forces."
As Kim emerged as a strong candidate for national security chief, the North's Committee for the Peaceful Reunification of Korea on Thursday urged Park to "consider carefully," calling the military "the most dangerous forces infatuated with fratricidal confrontation and war against the North."
Han, 63, was the JCS chairman in 2010-11 and deemed a top expert in policy and strategy.
In 2006, he led high-level talks with North Korea as director general for defence policy planning. He discussed ways to defuse cross-border tension in the border truce village of Panmunjeom with Kim Yong-chol, now director of the North's Reconnaissance General Bureau.
"(Kim Kwan-jin) is someone who dedicated his entire life to national security with his experiences in the field, operation and strategy," Min said.
"We think Han is the right person to ensure national security and the people's safety given his expertise in combat operations and policy and the military's strong confidence with him."
With their nominations, Seoul is expected to maintain the current line in its North Korea policy, focusing on beefing up deterrence amid concerns over a fourth nuclear test.
Criticism persists that Park's North Korea initiatives have been dominated by former military commanders in the presidential National Security Council such as Kim Kwan-jin, Nam Jae-joon and Kim Jang-soo, and thus lack flexibility and action plans.
Given the controversy, Park is widely expected to tap a nonmilitary official to head the spy agency, such as Lee Byung-kee, ambassador to Japan who served the NIS as deputy director, or Kwon Young-se, a former prosecutor and the current ambassador to China.