South Korea's special prosecutor investigating a corruption scandal involving President Park Geun-hye said on Sunday it had no choice but to raid the presidential offices, Yonhap News reported.

Special prosecutors are investigating allegations that Park colluded with a friend, Choi Soon-sil, and aides to pressure big companies to contribute to foundations set up to back her policy initiatives.

Prosecutors say they need access to the presidential offices as part of their investigation. The office has denied access.

Park has immunity from prosecution as long as she is in office even though her powers are suspended.

Yonhap gave no further details.