Roh committed suicide in 2009 amid a corruption inquiry into his family and aides, which the former liberal leader's associates called political retaliation. Moon served as Roh's chief of staff.

"If former President Lee cares about the country even a little, we strongly urge him to cooperate with the law enforcement authorities' investigation without losing his dignity as a former head of state," Choo Mi-ae, the leader of the ruling party, said during a party meeting.

Woo Won-shik, the party's floor leader, chafed at Lee's argument that the investigation into Lee's associates is part of a political ploy aimed at obliterating conservatives.

"I cannot help but take umbrage at former President Lee's unapologetic statement and attempt to dilute corruption allegations," he said. 

The conservative LKP doubled down on its claim that the ongoing probes that the Moon government has been carrying out under the name of "eliminating accumulated ills" were politically motivated.

"President Moon said yesterday that he cannot help but feel incensed ... we cannot help but feel deeply worried (about his remarks)," Kim Sung-tae, the LKP floor leader, said during a party meeting.

"The moment the president shows rage rather than composure, it would become political retaliation," he added.

The war of nerves between the rival parties has been intensifying ahead of the June gubernatorial and mayoral elections, which are seen as a bellwether for public sentiment towards the Moon administration. (Yonhap)