South Korea's main opposition party has filed a motion to impeach Prime Minister Han Duck-soo. Han has been serving as the acting President since President Yoon Suk-yeol was suspended from his duties.

The floor leader of the largest opposition Democratic Party, Kim Chan-dae, said: "Prime Minister Han is not eligible to serve as acting president. He has no intention to protect the constitution." The party says lawmakers are expected to vote on the motion as early as Friday.

This comes after Han said on Thursday he would not approve three nominees to the Constitutional Court for the time being.

The court will decide whether Yoon's impeachment is valid. It consists of nine justices, but three seats are vacant. It will hear the first preparatory hearing for Yoon's impeachment on Friday.

Meanwhile, former defense minister Kim Yong-hyun's legal team says their client maintains the martial law declaration issued earlier this month was lawful.

Kim has been indicted on insurrection charges.

Kim's lawyer, Yoo Seung-soo, said: "The martial law decree was intended to raise an alarm about what is going on in parliament. There were no casualties. The troops withdrew in an orderly manner and were under the commanders' control."

Investigators also sent a third summons to Yoon's office to appear for questioning on Sunday. Yoon did not respond to the previous orders. Local media say the authorities could take steps to detain the president if he continues to deny their requests.