Lawmakers are gathered at the National Assembly in western Seoul to convene a parliamentary plenary session on Thursday. (Yonhap)

The opposition-led National Assembly unilaterally passed a special bill on Thursday to open an investigation into suspicions that the Yoon Suk Yeol administration abused its power to disrupt a probe into the death of a young Marine last year.

Of the current total 296 members of the single-chamber National Assembly, all 168 who participated in the vote moved in favor of passing the bill, which seeks to launch an investigation into suspicions that the Yoon administration coerced an investigator to abandon charges against a division commander over the death of Marine Cpl. Chae Su-geun. Chae died after being swept away during a search and rescue operation in a flooded stream in July last year, in which he lacked safety equipment.

All ruling People Power Party lawmakers, except one, boycotted the move, leaving the afternoon plenary session where the voting took place.

The opposition's move to pass the bill poured cold water on the short-lived mood of reconciliation among Yoon and the rival parties, which had stemmed from Monday's first-ever meeting between the president and main opposition leader Lee Jae-myung.

The presidential office and the ruling party denounced the opposition's decision to pass the investigation bill into the death of the marine, calling it a political maneuver.

"We express strong regret over the (main opposition) Democratic Party of Korea's move to unilaterally pursue the special investigation bill on the death of Marine Cpl. Chae," Chung Jin-suk, the president's new chief of staff, said in a briefing.

"The Democratic Party's move to pass the bill is an example of bad politics in which they try to use the tragic death of Marine Cpl. Chae (for political gain)," he added.

Yun Jae-ok, the ruling party floor leader and acting chairman, refused to cooperate with the opposition in the near future, and revealed plans to officially request Yoon to exercise his veto power on the contentious investigation bill.

"We believe we can only ask President Yoon to exercise his veto power considering the legislative process (shown today) and the details of the (contentious) bill," Yun said in a press conference held after the ruling party lawmakers left the plenary session.

Earlier in the same plenary session, the rival parties passed a watered-down version of a special investigation bill that opens a new probe into the Oct. 29, 2022 Itaewon crowd crush disaster in a rare moment of bipartisan cooperation.

Of the 259 Assembly members that participated in the voting, 256 voted in favor of passing the bill, which seeks to launch an independent investigation panel for the tragedy that claimed 159 lives. Three lawmakers from the ruling party opted to abstain from voting.

The previous version of the bill was initially vetoed by President Yoon on Jan. 30 after the opposition-led parliament passed it on Jan. 9, with the ruling People Power Party boycotting the vote.

The new bill is an outcome of the rival parties each agreeing to make some concessions on the details in the clauses to find a middle ground, following the first-ever official meeting between Yoon and Lee.

A major change from the previous version of the bill was to omit clauses that give the panel the right to access several classified legal documents tied to the investigations on the Itaewon tragedy, coupled with the power to request a warrant wtih prosecutors if their request for such documents is rejected twice.

The panel was also downsized from the previous total of 11 members to 9 people. The members will also be named by the Assembly Speaker among candidates nominated by the rival parties, while the previous version also included nominees put forward by the bereaved families of the tragedy.

The panel will remain in power for one year following its launch but will be able to extend the period once for a maximum of three months.