South Korean prosecutors questioned Choi Soon Sil, the woman at the centre of the political scandal involving President Park Geun Hye. Choi faces allegations of fraud and meddling in state affairs.
Choi Soon Sil, who has close ties to Park, has been accused of abusing her personal connections with the president for influence and interfering in state affairs. However, Choi has denied any criminal wrong-doing.
"We hope that the various allegations can be thoroughly verified," presidential spokesman Jung Youn Kuk told AFP ahead of Choi's questioning at Seoul.
Park and Choi have been close friends for almost 40 years. The 60-year-old Choi is the daughter of a South Korean religious leader and one-time mentor of Park, Choi Tae-min.
On Sunday, Park carried out a partial reshuffle of her key aides and ordered her secretariat to hand in their resignations. The presidential spokesman said she has already accepted resignations from her chief of staff and four senior presidential secretaries.
Experts said the political scandal might paralyse Park's administration "underlining her lame-duck status ahead of presidential elections in December next year".
Lee Kyung-jae, Choi's lawyer said: "Choi told me she will cooperate with the investigation and expressed her deep apology to the people for letting them down and causing them frustration."
Meanwhile, the main opposition Democratic Party of Korea has stopped short of demanding Park's resignation. The part is refusing to begin any sort of cross-party talks until the investigation into Choi has run its course.
On Monday, presidential spokesman Jung said in his briefing that "the ongoing political uncertainty would not be allowed to open even the slightest crack in the country's defence posture".
"The government will maintain a firm readiness under any circumstances, and unwaveringly take care of key foreign policy and security issues," Jung added.