The logo of the Corruption Investigation Office for High-ranking Officials is seen in this photo taken on Jan. 23. Yonhap

South Korea's anti-corruption agency on Thursday transferred to prosecutors its case against impeached President Yoon Suk Yeol, recommending his indictment for insurrection and abuse of power over his brief martial law declaration.

The Corruption Investigation Office for High-ranking Officials (CIO) said it would ask prosecutors to indict Yoon over allegations he was the ringleader of an insurrection, for abuse of his authority and obstructing others from exercising their rights.

Yoon, impeached and suspended from power on Dec. 14, has been incarcerated since last week while investigators probe his Dec. 3 attempt to impose martial law - a move that shocked the nation even though it was overturned within hours by parliament.

The CIO was launched in 2021 as an independent anti-graft agency to investigate high-ranking officials including the president and their family members and has led a joint team involving police and the defence ministry, while prosecutors carry out their own probe.

Under the law, the CIO can only investigate and does not have the authority to prosecute the president and must refer any case to the prosecutors' office for further action.

The CIO has said Yoon's detention is due to end around Jan. 28, but they expect prosecutors to ask the court to extend it for another 10 days before they formally charge Yoon.

The Supreme Prosecutors' Office declined to comment.

Prosecutors have already charged Yoon's defense minister at the time, Kim Yong-hyun, with insurrection. Chiefs of the Capital Defense Command, the Defense Counterintelligence Command, and the Seoul police and the national police commissioner are also among the officials indicted so far.

Since his arrest on Jan. 15 — the first-ever for a sitting South Korean president — Yoon has refused to speak to CIO investigators and defied their summons.

Lee Jae-seung, deputy chief of the CIO, said it would be more "efficient" for the prosecutors to take over the investigation before indicting Yoon, citing the president's refusal to cooperate.

"Despite the fact that the suspect is under serious allegations that he was ringleader of an insurrection, he continues to be uncooperative to this day, not responding to the criminal justice proceedings and refusing our questioning itself," Lee told a briefing.

He said investigators had obtained testimonies from several military officials on Yoon allegedly trying to arrest politicians and mentioning a second martial law order. Yoon and his lawyers denied these allegations.

Impeached President Yoon Suk Yeol attends the third hearing of his impeachment trial at the Constitutional Court in Seoul, Jan. 21. Yonhap

Yoon, a top prosecutor before becoming president, now finds his criminal case in the hands of prosecutors from that same world, though how close their current ties are is unclear.

Yoon's lawyers have repeatedly said the CIO has no authority to handle his case as the law stipulates a wide-ranging list of high-ranking officials and violations it can investigate, but has no mention of insurrection.

They also said any criminal investigation should be conducted after the Constitutional Court decides whether to remove Yoon from office in its separate trial on his impeachment.

The lawyers reiterated the position on Thursday to say they will hold the CIO accountable for what they called its illegal investigation, while asking prosecutors taking over the case to comply with the law.

In comments on Tuesday to the Constitutional Court, Yoon denied ordering troops to drag lawmakers out of parliament or asking the finance minister to prepare a budget for an emergency legislative body.

Insurrection, the crime that Yoon may be charged with, is one of the few that a South Korean president does not have immunity from and is technically punishable by death. South Korea, however, has not executed anyone in nearly 30 years.

Yoon arrived at another Constitutional Court hearing in his impeachment trial on Thursday afternoon.

Former defense minister Kim is also expected to be called as a witness. (Reuters)