South Korea's ruling Saenuri Party on Monday cast doubt on the neutrality of National Assembly Speaker Chung Sye-kyun after he made controversial remarks during last week's parliamentary session to sack the agricultural minister.

Last week, South Korea's opposition parties passed a motion to dismiss the Cabinet minister over alleged ethical lapses, while the ruling party boycotted the vote and walked out of the session. The Saenuri and the presidential office have argued the dismissal motion was politically motivated and had nothing to do with actual wrongdoing.

In a voice recording obtained by the Saenuri, Chung, who is originally from the main opposition Minjoo Party of Korea, said the ruling party should not "ask something for free."

Political observers said the recording came after Chung unknowingly left his microphone on during the session.

"They (Saenuri) should bring up something, whether it be (the extension of the investigation on) Sewol or something else, but they are not doing it," Chung said in the recording to an unidentified person.

The Sewol ferry, which sunk on April 16, 2014, was en route to the southern resort island of Jeju from Incheon, west of Seoul, with a total of 476 passengers. Most of the 304 dead were high school students on a field trip.

The incident dealt a harsh blow to the government for its failure to properly rescue the passengers, as well as its negligence in overseeing the safety regulations of local ships. The opposition has tried to extend the probe as long as possible, while the government has objected to such a move, pointing out that the investigation's duration has officially expired.

"The recording is evidence that (the Minjoo) condemned an innocent minister as part of a bargaining ploy," the Saenuri said in a statement, adding Chung should resign from his current post.

Chung's political neutrality was also questioned earlier after he seems to have sided with the opposition bloc's stance on the deployment of an advanced U.S. missile defense system.

While the government claims the Terminal High Altitude Area Defense battery is important in defending the country from North Korea's provocations, the opposition parties claim that the move may make Beijing and Moscow uncomfortable, and should be reconsidered.

Chung, however, said the latest remark was only an expression of regret that the ruling and opposition parties failed to narrow their differences on key issues.

"Politics is about conversation and compromise," Chung said about the leaked recording, adding parties failed to come up with a solution due to the rigid stance of Saenuri lawmakers. (Yonhap)