South Korean media say prosecutors have detained former Defense Minister Kim Yong-hyun amid a probe into alleged treason after the president declared a short-lived state of martial law.

The Yonhap News Agency reported that Kim was detained on Sunday, after showing up for questioning by prosecutors earlier in the day to cooperate with the investigation.

Kim allegedly advised President Yoon Suk-yeol to declare martial law. Opposition party members have filed a complaint accusing Kim of treason and other misdeeds.

Media reports said Kim was detained after questioning by the prosecutors, who seized his mobile phone.

Kim expressed his intention to step down on Wednesday after the martial law episode, saying that he was responsible as defense minister for causing confusion and concern to the public. Yoon accepted his resignation the following day.

South Korean media say Kim and Yoon attended the same high school, with the former defense minister being Yoon's senior by one year. Kim was reportedly the closest aide to the president.