Yoon Chang-Jung, a former spokesman for Park, was sacked last week over allegations that he had sexually harassed a female intern -- a Korean-American -- while accompanying Park on her May 5-9 trip.
The presidential chief of staff today said Yoon had been involved in an "embarrassing and unsavoury incident" and apologised to the victim and her family.
"We offer our deepest apology to all South Koreans, the victim, her family and Korean expatriates overseas for the incident," Huh Tae-Yeol told a press conference, adding he had been left "speechless".
Huh added that Lee Nam-Ki, the top spokesman for Park and Yoon's superior, had offered to resign to take responsibility but did not say whether the resignation had been accepted.
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Yoon returned to Seoul on Wednesday without accompanying Park on her trip to Los Angeles, the final stop of her US visit.
Washington DC police told the South's Yonhap news agency they were "investigating the report of a misdemeanour sexual abuse" by an unnamed 56-year-old male suspect.
A Washington police report obtained by Yonhap and The Washington Post said the suspect allegedly "grabbed her buttocks without her permission".
"I implore her to forgive me if I had hurt her due to differences in culture. I offer my apology to her," the 56-year-old said at a live TV press conference yesterday.
The scandal overshadowed Park's US tour -- her first overseas trip since taking office in February -- that would otherwise have been considered a success.