The pan-opposition block is pressing the ruling camp to support their plan to launch an independent counsel probe into controversies surrounding a military report on a Marine's death last year.
The main opposition Democratic Party(DP) and five other opposition parties made the call on Friday in a joint news conference with a group of retired Marines.
They urged the ruling People Power Party to join the move to pass the special counsel bill before the term for the current 21st National Assembly expires in late May, saying that the public sentiment on the matter has already been revealed through the April 10 general elections.
The DP fast-tracked the special counsel bill to a plenary session on May 2, seeking to probe allegations of state interference in the military's investigation into the death of Marine Corporal Chae Su-geun last year.
The young Marine was swept away by a torrent during a search and rescue operation for flood victims in July of last year, and the then Defense Minister Lee Jong-sup's alleged interference in the military's case report sparked public criticism for seeking to protect Marine leadership.