South Korean President Yoon Suk Yeol's newly appointed national security adviser said he feels a grave responsibility in assuming the role at a critical time, a day after he was appointed to replace Kim Sung-han who stepped down amid rumors of protocol blunders.

Cho Tae-yong, who was a US ambassador, said he is given the mission of completing the Yoon administration's goal of turning South Korea into a pivotal state.

"I feel a great sense of responsibility to take on the position of the chief of the National Security Office at a critical time," Cho told a reporters' briefing Thursday.

He also stressed the importance of collaboration and unity among members of the presidential office, including the National Security Office.

Earlier in the day, Yoon also tapped First Vice Foreign Minister Cho Hyun-dong as the new ambassador to the US. The nominee is a career diplomat who took charge of South Korea's mission to North American countries and handled North Korean nuclear issues for years.

According to a senior official at the Foreign Ministry, Yoon's office will request the US administration's acceptance of Cho promptly, as the president is scheduled for a state visit to the US at the end of next month.

Agrement, a state procedure for receiving a foreign member on a diplomatic mission, takes about four to six weeks, according to reports. The official, expressing confidence in the transition, cited precedent of the US accepting a Korean envoy in two weeks upon request. The official also refuted concerns of an operational vacuum, adding that both the new security adviser and the ambassador nominee to the US have worked closely on Yoon's US trip.

The post of first vice foreign minister remains vacant, amid reports that other high-profile posts in Yoon's security and foreign affairs lineup are also subject to change. Yoon's office said there will be a swift decision on filling the foreign vice minister's position. Meanwhile, Foreign Minister Park Jin denied rumors that he would be leaving the post soon to prepare for a general election, and made assurances that he would stay put.

The latest reshuffle in Yoon's security and foreign affairs team came after the security adviser and his team members all tendered resignations.

Consecutive resignations from the team have raised suspicions that they were pressured to step down over repeated protocol gaffes.

According to the Chosun Ilbo, citing sources from the presidential office, the team omitted the White House's request of holding a joint performance between singer Lady Gaga and South Korean girl group Blackpink at a state dinner for a number of times since January. Yoon, who was briefed only in March, held the team responsible for the delayed response to the White House, it said.

A senior presidential official said on the condition of anonymity he understands that the replacement of Yoon's security adviser came as Cho was seen as "more suitable" to implement "detailed" diplomatic missions and strengthen the alliance between the US and Korea than Kim, who has an academic background.

Announcing his resignation, Kim said on Wednesday that he will be returning to Korea University where he taught before he joined Yoon's presidential campaign.

Yoon's office also said professor Park Cheol-hee from Seoul National University's Graduate School of International Studies was appointed as the head of Korea National Diplomatic Academy. Park replaces Hong Hyun-ik who was appointed by the previous liberal Moon Jae-in administration.