Acting President Han Duck-soo delivers a speech at the government complex in Seoul, Dec. 26. Yonhap

Acting President Han Duck-soo said Thursday he will not appoint justices to the Constitutional Court until the rival parties agree on whether he has the authority to do so before an impeachment ruling on President Yoon Suk Yeol.

Han made the remark in an address to the nation amid mounting pressure from the main opposition Democratic Party of Korea (DPK) to appoint justices to fill three vacancies at the court or face impeachment.

The National Assembly is set to approve three justice nominees later Thursday, a day before the Constitutional Court holds its first pretrial hearing on whether to uphold the Assembly's impeachment of Yoon over his brief imposition of martial law.

"It is the consistent spirit of our Constitution and laws that the acting president refrain from exercising the vital and exclusive authority of the president, including appointments to constitutional institutions," Han said from the government complex in Seoul.

"In the event it is inevitable to exercise such authority, I believe it has been an unbroken custom in our constitutional history that the ruling and opposition parties first reach an agreement at the National Assembly," he said.

The acting president earnestly pleaded with politicians, including National Assembly Speaker Woo Won-shik and the leaders of the ruling People Power Party and the DPK, to demonstrate "wisdom and courage" to break the political deadlock.

"I will defer the appointment of Constitutional Court justices until the ruling and opposition parties submit an agreed-upon measure," he said. "Once the ruling and opposition parties submit an agreed-upon measure, I will immediately appoint the Constitutional Court justices."