The center-left People's Party also demanded the education and labor committees open sessions to discuss allegations that a state agency gave job favors to Moon's son in 2006.

Floor leaders of the four parties plan to hold a meeting with Speaker Chung Se-kyun on Monday, in which Chung hopes to discuss pending parliamentary issues. During the talks, the Liberty Korea, Bareun and People's parties plan to request the opening of the committee meetings.

They recently increased pressure on Moon, capitalizing on the rekindled controversy over Seoul's abstention from a 2007 vote on a U.N. resolution against North Korea's human rights abuses when he served as a top presidential aide.

In a media interview published Friday, former Foreign Minister Song Min-soon reiterated that Seoul decided not to vote after consulting Pyongyang. Moon then served as the chief of staff to liberal President Roh Moo-hyun, who led the country from 2003-2008.

Song, the country's top diplomat from 2006-2008, first made the revelation in his memoir published in October.

Moon dismissed Song's claim, saying Seoul had contacted Pyongyang to "notify" it of the decision to abstain, not to hear its opinion on the issue.

Kim Seon-dong, deputy floor leader of the Liberty Korea Party, said the party will push to investigate the controversy and other allegations that former President Roh received bribes and Moon pressured a former military intelligence chief to lead efforts to abolish the draconian national security law.

Bareun Party floor leader Joo Ho-young said the party will demand the disclosure of information related to the 2007 vote. 

Joo Seung-yong, floor leader of the People's Party, held a news conference Sunday calling for the education and labor panels to look into the allegations about Moon's son.

In 2006, he got a job at a state labor information agency headed by a former presidential aide close to Moon. The allegation of preferential treatment was first raised in 2007, but the Labor Ministry concluded no special treatment had been extended.

But Moon's rivals have renewed the claims ahead of the May 9 election, saying his job application was altered during the investigation to cover up evidence of the preferential treatment.

The Democratic Party said it would not agree to open any parliamentary sessions as rival parties will try to use them to launch political attacks based on unverified allegations. (Yonhap)