The Seoul prosecutors' office summoned him late last month, but Choi refused to cooperate saying that it's not a fair probe.

When the prosecution asked him to appear the next day, he requested the questioning be rescheduled for Tuesday, which it accepted.

In such cases, prosecutors usually apply for an arrest warrant if the suspect is deemed to be avoiding the probe or a flight risk. But the prosecution needs parliament approval as an incumbent lawmaker like Choi has immunity from an arrest.

Choi, a four-term conservative lawmaker, is suspected of taking 100 million won ($92,000) in bribes from the National Intelligence Service when he was finance minister under Park's leadership in 2014.

Prosecutors believe the money was paid to lobby him for retaining the spy agency's off-book funds -- with which it allegedly paid Choi -- when then opposition parties pushed for scaling it down on the basis of the untraceable nature of the "NIS special activities fund."

Two former NIS chiefs have been arrested for arranging the illegal payments of at least 4 billion won from its coffers to then Park's two aides, who have also been arrested over bribery charges. Lee Byung-kee, one of the two arrested, apparently told the prosecution that he approved the NIS payment to Choi when he was the director in October 2014. 

Separately, Kim Tae-hyo, a Sungkyunkwan University professor who served as a senior secretary to former President Lee Myung-bak, appeared for questioning at 10:30 a.m., the office said.

Kim is suspected of involvement in the military cyberwarfare command's operations to write Internet comments to sway public opinion in favor of then ruling party candidate Park, in the runup to the 2012 presidential election.

"I will faithfully answer and explain myself based on facts,"

he told reporters.

Park won the election against her opposition Moon Jae-in, though she was removed from office earlier this year due to a massive corruption scandal involving one of her longtime friends. Moon succeeded her by winning the presidential election in May. 

Both Choi and Kim have denied any wrongdoing.

The investigation of Kim and other top presidential officials during the Lee administration is considered a step toward ultimately investigating the former president. Lee has recently categorically denied any knowledge or involvement in the military's alleged political interference. (Yonhap)