South Korea's presidential office apologised Sunday for an "very shameful" scandal involving a former senior official who was fired midway through President Park Geun-Hye's US visit.

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 Sunday 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".

"This is an unacceptable and very shameful incident," he said without elaborating on its nature.

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.

Yoon returned to Seoul on Wednesday without accompanying Park on her trip to Los Angeles, the final stop of her US visit.

Washington D.C. police told the South's Yonhap news agency they were "investigating the report of a misdemeanor 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".

Yoon denied the allegation, saying he had merely "patted her waist" while drinking with the intern -- a college student in her early 20s -- at a Washington bar Tuesday night and he had no sexual intent.

"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 on Saturday.

The affair overshadowed Park's US tour -- her first overseas trip since taking office in February -- that would otherwise have been considered a success.

North Korea said Sunday the scandal showed Park's trip had been a failure.

Its ruling party newspaper Rodong Sinmun, monitored by Yonhap, said in a commentary that a senior member of Park's entourage had committed a "shameful" act of indecency during the visit.

Park -- the South's first female president -- held a summit with President Barack Obama, with the two leaders united in a vow to offer no concessions when dealing with the North.

She also addressed a joint session of the US Congress, stressing that Pyongyang had to give up its nuclear weapons while also proposing small peace steps after months of high tensions.