Leaders of three opposition parties are set to put their heads together early this week to discuss ways to boost their push for a special probe into the sprawling online opinion rigging scandal involving a ruling party lawmaker, parliamentary sources said Sunday.

The leaders and whips of the Liberty Korea Party, Bareunmirae Party and the Party for Democracy and Peace will hold a meeting in the National Assembly on Monday, according to the sources.

They are expected to share views on the bills proposed by two of the parties seeking an independent counsel and a separate parliamentary inquiry into the alleged massive manipulation of Internet comments in online news articles about President Moon Jae-in. 

Prosecutors have charged a 48-year-old Internet blogger, who goes by the nickname "Druking," for using a software program to jack up the number of clicks in support of critical comments about the president.

The incident has sparked a fierce political dispute ahead of the June 13 local elections, as Rep. Kim Kyoung-soo of the ruling Democratic Party, a close aide to President Moon, is known to have been in contact with Druking for years.

The opposition parties stepped up their calls for an independent counsel, claiming the Seoul police agency has been conducting a lax probe because it involves the president and Kim.

The LKP submitted its own bill for a special probe into the scandal early last week, and the minor Bareunmirae Party is following suit. The minor Party for Democracy and Peace also pressed the DP to accept the demand for a special probe.

The ruling party said the parties should wait until the end of the police investigation. It views the demand for a special probe as a political offensive ahead of the local elections.

Should the opposition parties push ahead with the bills, the ruling party will likely be placed on the defensive. The three opposition parties hold a combined 160 seats, more than enough to pass the motion. 

The presidential office Cheong Wa Dae has yet to formally comment on the scandal.(Yonhap)