SEOUL — South Korea's opposition said on 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.

A political crisis gripped South Korea when President Yoon Suk-yeol, currently suspended, declared martial law on Dec 3.

Yoon was stripped of his duties by parliament on Dec 14 over the dramatic declaration, but a Constitutional Court ruling upholding the decision of 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 bench, something he has so far refused to do, essentially leaving both sides in a deadlock.

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

Han has said he would certify the judges' appointments only if his ruling People Power Party 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.

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

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

If the opposition succeeds in its bid on Friday, South Korea will see its second impeachment of a head of state in less than two weeks, further destabilizing its vibrant political scene.

As for Yoon, the nation's anti-corruption investigative body on Thursday asked him to appear for questioning on Sunday over his martial law imposition.

The Corruption Investigation Office for High-ranking Officials, or CIO, said on short notice that it delivered the third summons by express mail and e-mail to the presidential office and residence, calling for Yoon to appear at the CIO office at 10 am on Sunday.

Agencies - Xinhua