The main opposition party leader on Sunday threatened to impeach the prime minister this week, for allegedly accepting bribes in 2013 in a burgeoning graft scandal involving the Park Geun-hye administration's top officials.
Rep. Moon Jae-in, chair of the New Politics Alliance for Democracy, said accusations that Prime Minister Lee Wan-koo took 30 million won (S$37,000) from the late Sung Woan-jong justified an impeachment motion at the National Assembly. Lee has denied taking the money.
"Prime Minister Lee must resign. His involvement in the case is taking up his time and energy," Moon said. "That impedes him from running this country. This is also eventually harmful to the Park administration."
The comments come after President Park announced plans to make a decision on Lee after her return from a Latin America tour ending on April 27. Some governing Saenuri Party officials also back sacking Lee.
The Saenuri Party, meanwhile, said it opposes any motion for Lee's impeachment, and urged the opposition to wait until Park returns.
"If the prime minister were to step down immediately, an administrative vacuum is inevitable ... we should wait as the president has pledged to (decide on Lee) once she returns," Saenuri spokesman Rep. Park Dae-chul said.
Sung alleged that he gave Lee illegal campaign funds in the April 2013 parliamentary by-election during an interview with the Kyunghyang Shinmun on April 9, hours before police found him dead in an apparent suicide.
Sung was the former chair of construction firm Keangnam Enterprises and an ex-legislator with the Saenuri Party. Prosecutors had been probing him for allegedly swindling government subsidies in the weeks before his death.
In the interview, Sung accused prominent Park officials of taking illegal funds from him in the run-up to the 2012 presidential race. They include presidential secretaries, South Gyeongsang Province Gov. Hong Joon-pyo, and Lee, among others.
Authorities plan to summon past aides of Sung this week, to verify his accusations.
"It is hard to tell how this case will turn out. Although we may need a lot of time considering the way the probe is going, we may encounter help along the way if we continue to exert all efforts," an official was quoted as telling reporters on Friday.
Among those aides is Park Joon-ho, an ex-Sung aide and current chief of Onyang Hot Spring Hotel, a subsidiary of Keangnam Enterprises. Prosecutors suspect Park could be withholding testimonies supporting claims that Sung gave illegal funds to Lee
Prosecutors will also question 52-year-old Yoon, whose given name prosecutors did not publicize. Sung claimed that Yoon had delivered 100 million won to Hong in 2011 under his orders.
Hong has denied taking the money.
Prosecutors will seek testimonies from another aide to Sung surnamed Han, suspected to have knowledge of the whereabouts of illegal funds at Keangnam Enterprises amounting up to 3.2 billion won. Prosecutors think the money could have been used for bribes.
Sung's accusations sparked a public uproar this month against the Park administration and the Saenuri Party, as many of their top officials are alleged to have taken billions from Sung.
The unexpected scandal poses risks to the government's ability to implement its proposed changes to South Korea's public service pension, labour market, and initiatives aimed at encouraging entrepreneurship, as public attention sways lawmakers' focus to the scandal.
The scandal is also expected to shake the prospects for Saenuri Party candidates running in this month's parliamentary by-elections, set to contest four seats at the National Assembly.