The main opposition party in South Korea Thursday threatened to impeach Prime Minister Lee Wan-koo unless he resigns amid growing suspicions of his involvement in a bribery scandal.

"If he continues to hold onto his seat, our party will consider presenting a dismissal proposal," Moon Jae-in, the leader of the New Politics Alliance for Democracy (NPAD), told reporters in Ansan, Gyeonggi Province. "Our party will be forced to make a stronger decision if he does not step down for himself and the president does not take any action." Moon's remarks came as the ballooning bribery scandal involving eight politicians and presidential officials roiled the Park Geun-hye government and the ruling Saenuri Party, according to South Korea's (Yonhap) News Agency.

Before his suicide last week, Sung Wan-jong, a former head of a mid-size construction firm, claimed he gave up to several millions of dollars to eight politicians and former and incumbent presidential chiefs of staff, including 30 million won about (US$27,000) to the prime minister.

The opposition bloc and some ruling party lawmakers have called on Lee to step down amid a broadening prosecution probe into the scandal.