This combination of screenshots from YouTube shows Reps. Choi Soo-jin, left, and Kim Meen-geon of the ruling People Power Party sleeping while their fellow PPP lawmakers were staging a filibuster at the National Assembly in Seoul, Wednesday. Captured from YouTube

Reps. Kim Meen-geon and Choi Soo-jin of the ruling People Power Party (PPP) apologized Thursday after they were seen sleeping during a plenary session of the opposition-controlled National Assembly the previous day.

The incident was spotted while their own party was staging a filibuster in protest against the main opposition Democratic Party of Korea (DPK), which tabled a contentious bill aimed at mandating a special counsel investigation into the military's response to a Marine's death during a rescue operation in a flooded area last year.

"I apologize for causing trouble," Choi, who serves as a spokesperson for the PPP, said during a media briefing after a party meeting.

"I was exhausted, so I could not help dozing off during the session where my party was appealing for public help."

Kim also said she felt greatly embarrassed over the incident.

"I had to be on standby at the Assembly until late the night before. Many things happened simultaneously, which was very physically demanding," she said during a radio interview.

During the Wednesday session, National Assembly Speaker Woo Won-shik accepted the DPK's request to table the bill before an interpellation session. This prompted the ruling party, which has only 108 seats of the 300-member Assembly, to launch a filibuster.

The two PPP lawmakers were spotted sleeping right after Rep. Yoo Sang-bum began the party's filibuster as the first runner.

After this incident, the party leadership instructed its lawmakers to step out for a few minutes if they could not concentrate on the session or felt they would doze off.