SEOUL, Korea, Republic Of -- Disgraced South Korean President Park Geun-hye's longtime friend at the centre of a massive corruption scandal refused to testify at Park's impeachment trial on Tuesday, with lawmakers alleging that it was a stalling tactic.

The constitutional Court had expected to hear from Choi Soon-sil, a confidante of Park who's currently jailed and on trial herself for allegedly using her connections with the president to extort money and favours from companies and unlawfully interfere with government affairs. But Choi (pronounced Chwey) submitted documents to the court saying she was unable to testify.

Two jailed former presidential aides who purportedly helped Choi also refused to testify, saying they needed to prepare for their own trials.

Lawmakers, who function as prosecutors at the impeachment trial, raised suspicions that Park's lawyers were controlling the witnesses in order to stall.

"Three important witnesses all refused to testify, like they planned it ahead. We think there's an invisible hand at work," said lawmaker Lee Chun-suak.

The court said that it would call Choi and the two former presidential aides as witnesses again next week, and that it would take steps to forcibly summon them if they refused to appear again.

The no-shows by the three came after the court last week heard from only one of four former and current presidential aides it had called in as witnesses. Police are currently trying to locate two of Park's former aides after court employees failed to deliver subpoenas to them.

Park's powers have been suspended since Dec. 9, when South Korea's opposition-controlled parliament voted to impeach her.