Police said they were looking for Mayor Park Won-soon near a small hill in Seoul's Sungbuk neighbourhood where his mobile phone signal was last detected.

The phone was turned off, they said.

His daughter called police on Thursday afternoon and said her father left "a will-like" message before leaving their home about four to five hours earlier.

The daughter did not explain the contents of the message, an officer at the Seoul Metropolitan Police Agency said.

His daughter said she decided to call police because she could not reach her father on the phone, the officer said on the condition of anonymity.

About 300-400 officers and a drone were mobilised in the search.

Kim Ji-hyeong, a Seoul Metropolitan Government official, said Park had not shown up for work on Thursday for unspecified reasons and had cancelled his schedule, including a meeting with a presidential official at his Seoul City Hall office.

The reason for Park's disappearance was not clear. The Seoul-based SBS television network reported that one of Park's secretaries had lodged a complaint with police on Wednesday night over alleged sexual harassment such as unwanted physical contact that began in 2017.

The SBS report, which did not cite any source, said the secretary told police investigators that an unspecified number of other female employees at Seoul City Hall had suffered similar sexual harassment by Park.

MBC television carried a similar report.

Both the Seoul Metropolitan Police Agency and Park's office said they could not confirm the reports.

Park, a longtime civic activist and human rights lawyer, was elected Seoul mayor in 2011.

He became the city's first mayor to be voted into a third term in June last year.

A member of President Moon Jae-in's liberal Democratic Party, he has been considered a potential presidential candidate in 2022 elections.

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