South Korea moves to reassure allies, calm markets

"South Korea will carry out its foreign and security policies without disruption and strive to ensure the South Korea-US alliance is maintained and developed steadfastly," Han said, according to a statement from his office.

Acting president Han Duck-soo says South Korea will ensure its US alliance is maintained. (AP PHOTO)

In a further attempt to stabilise the Asian nation's leadership, the main opposition party announced it would not seek to impeach Han for his involvement in Yoon's December 3 martial law decision.

"Given that the prime minister has already been confirmed as acting president and considering that excessive impeachments could lead to confusion in national governance, we have decided not to proceed with impeachment procedures," Democratic Party leader Lee Jae-myung told reporters.

Prosecutors said Yoon did not appear on Sunday morning in response to a summons for questioning in a criminal investigation into his martial law decision, and they promised to issue another order, Yonhap news agency reported.

Yoon and a number of senior officials face potential charges of insurrection, abuse of authority and obstructing people from exercising their rights.

The prosecutor's office did not answer requests for comment.

Han, a longtime technocrat picked by the conservative Yoon as prime minister, was elevated to acting president per the constitution while Yoon's case moved to the Constitutional Court.

Demonstrators seeking Yoon's ouster braved the cold to throng the streets outside the National Assembly building where he was impeached. 

Yoon's surprise martial law declaration and the ensuing political crisis spooked markets and South Korea's diplomatic partners, worried over the country's ability to deter nuclear-armed North Korea.

Biden told Han the ironclad US-South Korea alliance remained unchanged and Washington would work with Seoul to further develop and strengthen the alliance as well as trilateral co-operation including neighbour Japan, Han's office said.

The White House said in a statement that the US president "expressed his appreciation for the resiliency of democracy and the rule of law in the ROK", using the abbreviation for the country's formal name, the Republic of Korea.

Crowds seeking Yoon's ouster braved the cold to gather outside the building where he was impeached. (AP PHOTO)

Han convened his cabinet and National Security Council shortly after Saturday's impeachment vote and vowed to maintain military readiness to prevent any breach of national security.

He spoke by phone with the commander of US Forces Korea, expressing concern about the possibility North Korea could attempt military provocations, such as launching ballistic missiles or cyber attacks, Yonhap said, citing Han's office.

South Korea's partners wanted to see a credible and constitutional temporary leadership put in place as soon as possible, said Philip Turner, a former New Zealand ambassador to South Korea.

"They will be pleased to see Prime Minister Han take over as acting president," he said. 

"He is capable, experienced and well respected in foreign capitals."

But even with an acting president in place, international partners face months of uncertainty before a new president can be elected and a new government established, Turner said.

The Constitutional Court has up to six months to decide whether to remove or reinstate Yoon. 

If he is removed or resigns, a new election will be held within 60 days.

South Korea's financial authorities vowed on Sunday to act as needed to stabilise markets while the finance minister said he would announce an economic policy plan by year's end.