South Korean investigators sought an extension of a warrant to arrest impeached President Yoon Suk Yeol, as visiting US Secretary of State Antony Blinken reaffirmed confidence in the key Asian ally's handling of the political turmoil.

Investigators probing Yoon over his December 3 martial law proclamation failed to serve an arrest warrant on Friday, blocked by presidential security service guards who formed a human chain to prevent access to Yoon.

The arrest warrant, the first for a sitting president, was due to expire at midnight on Monday (1500 GMT). Investigating authorities said in a statement on Monday they had requested an extension of the warrant in court, but declined to say how long the extension will last.

Yoon, who has been impeached over his martial law declaration, is also under criminal investigation for possible insurrection.

The brief six-hour martial law declaration plunged one of Asia's strongest democracies into uncharted territory with the impeachment of not only Yoon, but also the prime minister who become acting president.

Yoon’s action drew a rare rebuke from officials in Washington including Blinken's deputy, Kurt Campbell, who has said it was “badly misjudged.”

Speaking after his meeting with South Korean Foreign Minister Cho Tae-yul, Blinken said Washington had expressed “serious concerns” to Seoul over some of the actions Yoon took over the course of his martial law declaration.

But the events unfolding since then have reflected South Korea’s brand as a democratic success, “which is remarkably strong,” Blinken said at a news conference.

“The response that we’ve seen, and that we expect to continue to see, is one that is peaceful and fully consistent and in accordance with the constitution and the rule of law.”

Yoon was impeached by parliament on December 14 and suspended from presidential duties. The Constitutional Court is trying the case to decide whether to remove him from office permanently or reinstate him.

The Corruption Investigation Office for High-ranking Officials (CIO), which is leading the criminal insurrection probe into Yoon, had sent a notice to police requesting them to take over execution of the arrest warrant.

A police official said at a news briefing that police believed there was a legal dispute over such a transfer and would discuss it with the CIO.

Yonhap news agency cited a police official as saying the arrest warrant will now be executed under the authority of the police joint investigation team and the CIO.

Yoon’s lawyers have argued that the CIO anti-graft force has no authority under South Korean law to investigate any case involving insurrection.

On Monday, Seok Dong-hyeon, a lawyer advising Yoon, said the bid to transfer the execution of the arrest warrant is effectively an admission by the CIO that its probe and the warrant were “illegal”.

The unprecedented attempt to arrest an incumbent president has intensified duelling rallies by those supporting Yoon, with the “Stop the Steal” slogans popularised by US President-elect Donald Trump voters, and those calling for Yoon’s punishment.

On Monday, a group of hardcore Yoon supporters led by Christian pastor Jun Kwang-hoon held a news conference and described the fight for Yoon as an “international battle” for freedom.

“Sadly, there's no Fox News in Korea,” it said in a statement, referring to the American cable news channel popular with Trump supporters.

Jun said Yoon supporters would continue rallies outside his residence until they “reap the results”.