A woman holds a lit candle at a protest in Seoul on Saturday to call for South Korean President Park Geun-hye to step down. [Photo/Agencies]

South Korean prosecutors said on Sunday that they believe President Park Geun-hye was an accomplice in a corruption scandal that has rocked her administration, in a heavy blow to her fight for political survival.

The prosecutors' comments, which came as they indicted a close friend of Park's and two of her former aides, are likely to spur stronger calls for her to step down or be impeached.

Park's close friend Choi Soon-sil and former presidential aide An Chong-bum were charged with abuse of power in pressuring companies to contribute funds to foundations at the center of the scandal, said Lee Young-ryeol, head of the Seoul Central District Prosecutors' Office.

"The special investigation team concluded that based on the evidence secured to date, the president was in complicity with Choi Soon-sil, An Chong-bum and Jeong Ho-seong to a considerable degree," Lee told a news conference.

Jeong, also one of Park's former aides, was indicted on a charge of leaking classified information to Choi.

Park cannot be indicted because she has constitutional immunity, Lee said, but added, "We will continue to investigate the president," without elaborating.

Under the constitution, a sitting president cannot be indicted unless on charges of treason.