Opposition parties on Monday ramped up their offensive against President Moon Jae-in over his prime minister nominee, pressuring him to directly apologize over the designate dogged by a string of allegations involving his family.

Observers, however, said that Moon's apology, should it be made, could help address the political standoff and pave the way for a parliamentary vote on Prime Minister-designate Lee Nak-yon this week.

The opposition bloc's criticism centered on Moon's failure to deliver on his campaign pledge to exclude anyone involved in five kinds of irregularities -- tax evasion, false address registration for personal gain, real estate speculation, draft dodging and plagiarism -- from public office.

Lee has been under fire for his wife's use of a false address to get assigned to a school in southern Seoul as a teacher. During the confirmation hearing last week, Lee acknowledged it, which triggered a public apology from Moon's chief of staff.

Chung Woo-taik, the floor leader of the main opposition Liberty Korea Party, repeated its demand that Moon come forward and explain whether he would stick to his professed nomination principles.

"Rather than making flimsy excuses through his chief of staff, (Moon) should confidently stand in front of TV (to explain his stance on the controversies) in the same way he did when he announced his election pledge," Chung said during a meeting of senior party officials.

He also renewed his call for the president to withdraw his nomination of anyone involved in the five irregularities to "lay the foundation for stable state management and genuine cooperation with parliament."

Seeking to defuse tensions with the opposition bloc, Jun Byung-hun, Moon's chief secretary for political affairs, said that the presidential office would exclude anyone who used a false address after July 2005 when the parliamentary hearing system was introduced from the list of Cabinet nominees.

Chung, however, dismissed it as "inappropriate."

Joo Ho-young, the floor leader of the splinter conservative Bareun Party, demanded that Moon apply as strict nomination principles as those his party had applied while it served as the main opposition party.

"The president broke (his promise) just two weeks after his inauguration, even though the nomination principles were included in his election manifesto," Joo said.

Progressive Justice Party leader Sim Sang-jeung also called on the president to seek the opposition parties and citizen's "understanding" for his nominee.

"That is the way (the president) can quickly normalize state affairs," Sim said during a party meeting.

"The opposition parties take issue not with the nominee's qualifications themselves, but with the president's breach of its earlier pledge ... The parties have the right to demand the president's explanation of this on behalf of citizens," she added. (Yonhap)