SEOUL: A US bribery case against two relatives has cast a pall over former United Nations chief Ban Ki-moon's planned return this week to South Korea, where he is expected to launch a bid to run for president.

Ban (72), has not declared his candidacy but has had a team of people laying groundwork in Seoul ahead of his planned arrival in South Korea on Thursday afternoon.

The former foreign minister consistently polls as a top candidate as South Korea braces for the possibility of an early election following parliament's December impeachment of South Korean President Park Geun-hye in an influence-peddling scandal.

If South Korea's Constitutional Court upholds the impeachment, Park would become South Korea's first democratically elected leader to leave office in disgrace.

Lee Do-woon, Ban's spokesman, was quick to distance Ban from the indictment, which was filed against his younger brother and his nephew in a Manhattan federal court. It accuses Ban's relatives of a scheme to bribe a Middle Eastern official for an attempted $800 million sale of a building complex in Vietnam.

"Ban was greatly surprised by the news, which he learned from the media. He knows nothing about it," said Lee, who also said Ban would address various concerns directly upon his arrival in South Korea.

Ban will take at least two weeks to decide whether he will run for president instead of taking to the campaign trail immediately, as many analysts and observers had expected, Lee told a media briefing.

Ban's spokesman also asked his supporters to refrain from greeting him on his arrival at Incheon airport.

Ban was not immediately available for comment. While not yet declaring an intention to run for president, he has said he would devote himself to the country after his UN tenure ended.

"MUST BE CONVINCING"

"What he needs to do is draw a line and say he has nothing to do with (the alleged improprieties), and he needs to be convincing in his explanation. Otherwise people will start doubting him," said Kim Sang-jin, professor of political science at Konkuk University.

"If he mishandles this people will start thinking that he is too weak or isn't as clean as they thought he was," Kim said.

Ban does not belong to any political party and has been out of the country for most of the past decade. However, a party, funds and political machinery to support him could come together quickly if and when he announces he will run for president on what is widely expected to be a conservative platform.

He had until recently been tipped to run as a member of Park's Saenuri party. However, being a Saenuri candidate looks far less attractive now because of the corruption scandal and he has been seen as likely to join with a new breakaway group from the conservative bloc.

The former UN chief's spokesman declined to comment on which party Ban might join.