South Korea's top presidential adviser on diplomacy and security has announced he is stepping down.

National Security Adviser Kim Sung-han said in a comment to the media that his resignation was effective Wednesday.

Kim said his goals of establishing a foundation to restore the alliance with the United States, improving relations with Japan, and strengthening security cooperation between South Korea, the US and Japan have been met "to some extent."

He said he believes his successor will be able to take over his role smoothly.

Kim did not provide any reason for his resignation.

South Korean President Yoon Suk-yeol has nominated the ambassador to the US, Cho Tae-yong, as Kim's successor.

Kim previously worked as a university professor of US diplomatic policy and other issues.

He has served as Yoon's national security adviser since the inauguration of his administration.

Kim accompanied Yoon to his visit to Japan earlier this month and other overseas trips.

South Korean media are reporting that officials at the presidential office have been replaced due to a protocol blunder concerning the president's US visit next month. Kim's resignation is rumored to have come against such a backdrop.

The sudden resignation of the top security adviser has prompted public concerns, as North Korea continues its ballistic missile launches and other provocations.