South Korea's opposition said Thursday it filed an impeachment motion against acting president Han Duck-soo, in an escalating row over the composition of the Constitutional Court which would decide whether to remove his predecessor from office.

South Korea fell in a political crisis when President Yoon Suk Yeol, currently suspended, declared martial law on 3 December.

Yoon was stripped of his duties by parliament on 14 December over the dramatic declaration, but a constitutional court ruling upholding the decision by lawmakers is necessary to complete the impeachment process.

The court is however currently short of three judges. While it can go ahead with its six members on the bench, a single dissenting vote would reinstate Yoon.

The opposition wants Han to approve three more nominees to fill the 9-member bench, something that he has so far refused to do, essentially leaving both sides in a deadlock.

The opposition Democratic Party therefore says the acting president should be impeached too.

"We have filed the motion… and will report it to the plenary session today," MP Park Sung-joon told reporters at the National Assembly of the action against Han.

"We will put it to a vote tomorrow."

Han's refusal to formally appoint the three judges proves that he "does not have the will or qualification to uphold the constitution," the Democratic Party's floor leader Park Chan-dae told reporters.

Han has said that he would certify the judges' appointments only if his ruling People Power Party (PPP) and the opposition reach a compromise on the nominees.

"The consistent principle embedded in our constitution and laws is to refrain from exercising significant exclusive presidential powers, including the appointment of constitutional institutions," Han argued.

"A consensus between the ruling and opposition parties in the National Assembly, representing the people, must first be reached," added the 75-year-old career bureaucrat. 

If the opposition passes the impeachment motion against Han in Friday's vote, it would mark the first time democratic South Korea has impeached an acting president.

In Han's place, Finance Minister Choi Sang-mok would step in as acting president.

Duty violated

In the impeachment motion, the opposition accuses Han of violating his duty as acting president, citing his refusal to formally appoint the judges and promulgate two special probe bills to investigate Yoon's short-lived imposition of martial law and graft allegations involving his wife, Kim Keon Hee.

Earlier this week, Han rejected the opposition's demand for the special bills that would establish two independent investigative bodies to probe the first couple, prompting warnings of impeachment from the Democratic Party.

Han is "intentionally avoiding the special investigation to probe those involved in the insurrection and has clearly stated his intention to reject the appointments of three Constitutional Court judges," the motion reads.

Such actions, it adds, are "in violation of a public official's duty to uphold the law... and serve the public."