SEOUL - South Korean investigators will seek 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 Dec 3 martial law on Friday again failed to serve an arrest warrant, 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, expires at midnight on Monday (1500 GMT). The Corruption Investigation Office for High-ranking Officials (CIO) plans to request an extension of the arrest warrant in court on Monday.

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

READ MORE: S. Korea's presidential security chief defies Yoon arrest bid

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 became acting president, and the appointment of a second acting leader.

Yoon's actions 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 been a reflection of 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."

ALSO READ: S. Korean Army chief, special warfare commander indicted in martial law probe

Yoon was impeached by parliament on Dec 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 CIO, which is leading the criminal insurrection probe into Yoon, has sent a notice to police requesting them to take over the 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 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 accusations.

ALSO READ: S. Korean investigators leave Yoon's residence after failing to arrest him

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 dueling rallies by those supporting Yoon, with the "Stop the Steal" slogans popularized 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".