The presidential office says it regards the leaking of phone talks held between President Moon Jae-in and U.S. President Donald Trump as a serious matter.

A diplomat at the South Korean Embassy in Washington had told main opposition Liberty Korea Party lawmaker Khang Hyo-shang about the two leaders' phone conversation that took place early this month.

A ranking presidential office official told reporters Thursday that the top office considers it a "leak of state secrets" and that the person responsible has also admitted to doing so.

The official added, however, that the leak does not constitute tipping off information or whistle-blowing aimed at exposing irregularities and corruption of an organization.

The official said this matter can even damage trust between Seoul and Washington and adversely affect Korean Peninsula affairs at a sensitive time in regional security.

Regarding the alleged leaker, the official said the Foreign Ministry plans to investigate and will take appropriate steps accordingly.