Sources close to South Korean President Yoon Suk-yeol say the president is unlikely to appear for questioning as requested, for the second time, by a joint investigation team of authorities.

The investigation team, including police, is seeking to question Yoon on suspicion of insurrection and other offenses in relation to his declaration of martial law early this month.

It sent its second summons asking Yoon to appear for questioning at 10 a.m. on Wednesday.

A lawyer involved in the selection of Yoon's defense team said on Tuesday that the president was unlikely to comply with the request. Yoon did not heed a first summons that asked him to appear on December 18.

The lawyer noted that Yoon places priority on the Constitutional Court proceedings of his impeachment trial, which starts on Friday.

South Korea's Yonhap News Agency quotes a legal expert as saying that the joint investigation team may make a third request for Yoon's appearance if he does not comply with the second.

The expert reportedly says the team may consider filing for a court warrant to detain Yoon if he fails to comply the third time.

South Korea's Constitutional Court is due to hold a judges' meeting on Thursday ahead of the start of the impeachment trial.