The man central to blocking investigators from arresting South Korea's impeached President Yoon Suk Yeol has come under the spotlight himself in the political crisis sparked by Yoon's brief declaration of martial law last month.

South Korea’s President Yoon Suk Yeol again declined to appear for questioning by investigators over his martial law declaration as the embattled leader seeks to first defend himself at an impeachment trial.

The opposition said the impeached president’s letter to his supporters was aimed at triggering an ‘extreme clash’.