South Korea's opposition lawmakers have filed a motion to impeach the country's prime minister and acting leader Han Duck-soo, less than two weeks after parliament voted to impeach President Yoon Suk Yeol.

This comes after Han refused to appoint constitutional court judges nominated by the main opposition Democratic Party (DP).

"Han has revealed himself to be an acting insurrectionist, not an acting president", DP's floor leader Park Chan-dae said on Thursday.

The opposition has also accused Han of aiding Yoon's martial law attempt on 3 Dec. Han earlier apologised for failing to block it.

Han also vetoed several opposition-led bills, including one that proposed a special investigation into Yoon's short-lived martial law declaration.

The impeachment motion is expected to be put to a vote in the next 24 to 72 hours.

For it to succeed, 151 out of 300 MPs must vote for it.

The DP currently holds 170 of the 300 seats in parliament. The opposition bloc together holds 192 seats.

Opposition parties had been hoping Han would not stand in their way while acting as the country's caretaker president, and that he would allow bills to pass.

But instead, he has held firm, deepening the political strife.

On Tuesday, Han concluded a cabinet meeting without reviewing the two opposition-sponsored bills that called for special counsel investigations into the martial law declaration and corruption allegations involving First Lady Kim Keon Hee.

He said he did not put them on the agenda so as to give the ruling and opposition parties more time to reach a compromise.

But DP's floor leader Park Chan-dae slammed him for "buying time and prolonging the insurrection".

"We've clearly warned that it's totally up to Prime Minister Han Duck-soo whether he would go down in history as a disgraceful figure, as a puppet of rebellion plot leader Yoon Suk Yeol, or a public servant that has faithfully carried out the orders by the public," Park said in a televised party meeting.

And on Thursday, Han said he would not appoint the three judges the opposition-dominated National Assembly had nominated to the constitutional court - which is deliberating whether Yoon should be impeached - unless the rival parties reach a consensus.

To this, Park said "it has become clear that Han Duck-soo is neither qualified nor willing to defend the constitution", adding that the opposition would "immediately" table the impeachment bill.

Han's ruling People Power Party said the opposition's threats have interfered with Han's "legitimate exercise of authority", while a senior official at the prime minister's office criticised the threats as "extremely regrettable".

Han stepped in as caretaker president after Yoon was ousted from office earlier this month. If lawmakers vote for Han to be impeached, Finance Minister Choi Sang-mok will be next in line.

This latest development in the country's political turmoil comes as the Seoul Constitutional Court is deliberating on whether Yoon should be permanently barred from office.

The court is expected to hold its first public hearing later this week.

It is unclear if Yoon himself will take the stand during the hearings, but protesters have vowed to keep up their calls for Yoon's removal during court proceedings.

Yoon is also under investigation for alleged insurrection over his failed attempt to put the country under martial law.

He has refused to accept several summonses delivered to him, and investigators have warned that they may issue an arrest warrant if he continues to be unresponsive.

Several senior officials - including former defence minister Kim Yong-Hyun, former interior minister Lee Sang-min and army chief Park An-su - are also being investigated.