Opposition leader Lee Jae-myung will hold a previously unannounced press conference at 11 a.m., officials said, as a prosecution investigation closes in on him after an arrest warrant was sought for one of his closest aides on charges of taking illegal campaign funds.

Kim Yong, a long-time confidant of Lee, is suspected of receiving 847 million won ($593,552) in illegal political funds last year from real estate developers involved in a corruption-laden project in the city of Seongnam when Lee was mayor.

Prosecutors suspect the money was used to help finance Lee's presidential campaign.

Lee, chairman of the main opposition Democratic Party, has denied any wrongdoing, saying he never used a penny of illegal money and he believes Kim is innocent.

Lee and his party have denounced the prosecution investigation as a political reprisal and an attempt to suppress the opposition. (Yonhap)