South Korea's leadership crisis deepened on Sunday as prosecutors named President Yoon Suk Yeol as a subject of a criminal investigation over last week's martial law attempt, a media report said, and his former defence minister was arrested.

South Korean President Yoon Suk Yeol narrowly escaped an impeachment attempt following his controversial martial law declaration earlier this week.

Most of Yoon's ruling party lawmakers boycotted a parliamentary vote Saturday to deny a two-thirds majority needed to suspend his presidential powers.