South Korea's Yoon willing to represent himself in legal proceedings: lawyer

South Korea's impeached President Yoon Suk Yeol is willing to present his views himself during legal proceedings related to his short-lived declaration of martial law, a lawyer advising Yoon said on Thursday.

Seok Dong-hyeon, a lawyer and long-time friend of Yoon, also told reporters that multiple - seemingly overlapping - investigations involving Yoon should be streamlined.

Yoon, who is an ex-prosecutor, shocked the nation on Dec. 3 when he declared martial law in a late-night televised address, before backing down hours later after outraged lawmakers rejected his decree.

"He (Yoon) has already apologised for surprising and shocking the public, and he remains apologetic for that... but he has a clear and confident position on the issues that are in dispute and must be addressed," Seok told a briefing, adding Yoon had never even considered insurrection.

Yoon was impeached by parliament in a vote last Saturday over his imposition of martial law, and faces a Constitutional Court trial on whether to remove him from office or restore his presidential powers.

Yoon also faces investigations into whether the martial law declaration constituted insurrection, one of the few charges for which a South Korean president does not have immunity from.

"What kind of insurrection follows parliament's order to stop... and quits after two to three hours?" said Seok, who was wearing a souvenir watch from Yoon's presidential office.

Asked whether the martial law decree breached the constitution, Seok said Yoon viewed the situation of an overbearing opposition party controlling parliament, cutting the government budget and impeaching government officials as an "emergency state" and the basis to invoke martial order.

National Assembly speaker Woo Won-shik questioned such justification and called it contradictory to public sentiment against the martial law imposition.

"I think it was president Yoon's misjudgement," Woo told foreign correspondents on Thursday, accusing Yoon of doing little to communicate with parliament and not attending the opening of the new assembly nor addressing the parliament on budget unlike predecessors.

Yoon, who has not commented publicly since shortly after his impeachment, was currently staying at his residence with his duties suspended due to parliament's impeachment, Seok said.

While Yoon had apologised for rattling the nation with his martial law declaration, Seok also called for public understanding over the president's "pain" dealing with the opposition's excessive power.

It remained unclear if or when Yoon might comply with the investigations or respond to attempts to summon him.

Yoon is still forming legal teams to respond to criminal investigations into the martial law case and the upcoming impeachment trial at the Constitutional Court, Seok said.

The presidential office's security service said on Wednesday it would not comply with attempted raids by investigative agencies, according to Yonhap news agency, and the Constitutional Court said on Thursday the trial documents it tried to send to Yoon had yet to reach him.

Meanwhile, South Korea's acting president Han Duck-soo spoke with Japanese Prime Minister Shigeru Ishiba by phone on Thursday. They agreed to continue three-way security cooperation with Washington while facing growing North Korea-Russia military ties as well as Pyongyang's nuclear threat, Han's office said.

Domestic political uncertainty has increased pressure on the South Korean won. The currency hit its weakest level in 15 years on Thursday, also due to the US Federal Reserve's cautious stance on more interest rate cuts.

The investigations into the martial law declaration have also embroiled multiple officials and security personnel.

Former army intelligence commander Noh Sang-won was arrested late on Wednesday over his alleged involvement in Yoon's martial law order, a police official said on Thursday, becoming the latest high-ranking military officer detained.

In a fresh twist to the saga, Noh is accused of discussing military deployment plans with incumbent army officers at a fast-food burger joint, two days before Yoon declared martial law, the official said.

A lawyer who has represented Noh declined to comment and Reuters could not immediately reach Noh for comment.